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Purpose of this thread : My goal in starting this thread is to build a Glossary for useful terms which you will find recurring in any or all of the other forums.  Not quite Wikipedia but I'm sure you get the general idea.
term : definition

For example,
sweet potato: TP : a plant grown for its edible tubers  [note that TP (tender perennial) describes the plant under typical UK growing conditions.]

Please let a mod know if you would like us to add to the glossary.

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air layering : a propagation technique whereby compost is held in place above ground level on a suitable part of the parent plant.  Typically polythene and/or tape is used to hold a ball of compost in place until new roots are formed

allium : a family of plants which includes onion, shallot, garlic, chives and leek

allotment : a plot of land, usually rented or leased for the purpose of growing fruit and / or vegetables.  Historically, allotments were provided by farms/estates for their workers to grow food for their families.  The practice spread into towns and cities as the workforce migrated into urban settlements. See Clearing a New One

annual : a plant which requires to be planted from seed (or self-seeds) every year.  Compare biennial and perennial

Asteraceae: See Compositae .

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basal cuttings : a method of propagation of certain plants, e.g. dahlia, where early spring growth is rooted to form a new plant.  See http://www.chat.allotment-garden.org/viewtopic.php?p=74906#74906

* fallow : a technique to control annual weeds.  Best suited to later crops.  Prepare ground well in advance in the usual way but do not plant anything.  Allow annual weeds to flush (germinate) and simply hoe them off before planting your intended crop.

bhindi : see okra

biennial : a plant which requires two growing seasons to produce crop or flower, e.g. parsley which (although it can be picked in the first year) produces flowers and seeds in the second year.  Compare annual and perennial

black spot : a widespread disease (Diplocarpon rosae) of roses affecting lower leaves and eventually causing some defoliation

blight : one of several water moulds (oomycete) which affect primarily potatoes and tomatoes.  Late Potato Blight (Phytophthora infestans) is the most serious and most prevalent but there is also an Early Potato Blight (Alternaria solani).  Both are airborne.  For prevention/control of Late Potato Blight click here

bolting : describing a plant which wants to produce seed rather than edible crop.  Typically a tall flowering spike will emerge and all of the plants energies go into this.  Unless you wish to save the seed (but see F1 hybrid), then the plant has basically fulfilled its useful role and can be removed.

Bordeaux Mixture : a mixture of copper sulphate and slaked lime used to control blight in potatoes and tomatoes.  Approved for organic use as a last resort (according to www.organiccatalog.com)

bract : name given to a leaf which is modified to give a flower, as in e.g. poinsettia and arums

brassica : a family of plants which includes cabbage, cauliflower, brussel sprouts, broccoli, kale, mustard, radish, turnip, kohl rabi, wallflower, swedes, turnips, oilseed rape

broadcast : a technique for sowing seeds whereby seed is scattered over an area of ground rather than sown in rows

bull neck : of onions : The neck is swollen which prevents them from storing well.  Caused by too much nitrogen in the soil, over feeding - especially late in the season.  Use affected bulbs imediately after harvesting.

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calomel : mercury chloride : a toxic chemical still widely used as a fungicide and insecticide.  Certainly not organic.

catch crop: a quick growing crop (e.g. radish) that is grown alongside, or between successive plantings of a main crop.

cheshunt compound : ammonium carbonate + copper sulphate : a fungicide widely used to prevent damping-off in seedlings.  Not organic.

chitting : (of potatoes) : promoting strong early growth by arranging tubers in a single layer, e.g. on a tray.  Leave in a cold place in full indirect light

chitting : (e.g. sweet corn) : pre-sprouting by placing seeds on damp kitchen towel.  Re-plant into pots or final position when sprouts appear.  Can also be done with beans and peas

cloche : any kind of (transparent) low-lying temporary shelter for use on open ground.  Traditionally a glass bell or simply panes of glass arranged in 'tent' fashion using a special clip.  Now more commonly made of polythene with wire supports / fixings.

clubroot : a persistent soil-borne fungal disease affecting all brassicas and wallflowers.

coldframe : a low, glass-covered structure to provide sheltered growing conditions.  Sides may be timber, plastic or glass.

Compositae: a major group of plants which includes lettuce, chicory, endive, globe artichoke, cardoon, jerusalem artichoke, globe artichoke, aster, dandelion.  Also known as Asteraceae.

compost : a growing medium and soil enhancer produced by the breakdown of organic matter

coriander : HA or HB : a herb (leaf) and a spice (seed) used extensively in the cuisines of many countries.  Thai cookery also uses the root. Growing instructions for leaf

cotyledon; The primary leaves of an embryo that are present in the seed. These are the very first leaves that emerge and are not true leaves but serve to nourish the new sprout.

crop rotation : See rotation

cucurbit : mostly HHA or TA : a family of plants which includes marrow, courgette, melon, squash, loofah, gerkhin, gourd and pumpkin

cuttings : a method of propagation whereby sections of the parent plant are removed for rooting to take place. Not suitable for all plants. How to: hardwood, How to: softwood

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damping down : raising the humidity in a greenhouse by watering the floors and/or staging.  Tends to lower the temperature & reduce water loss from plants

damping-off : death of seedlings due to overly wet, crowded or poorly ventilated conditions

deadheading : manual removal of any spent, faded, withered or discoloured flowers from roses, shrubs, etc. over the flowering season.  Prolongs flowering period.

deep bed: an unconstrained bed which is created by adding soil / compost to raise the ground level.  Compare raised bed.

Derris : an insecticide of plant origin & so technically organic.  It is non-poisonous to warm-blooded animals but deadly to virtually all fish and insects (including beneficial ones).

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earthing up : the practise of drawing up loose earth around the stems or even over the foliage (esp. potatoes) of plants to improve the crop and/or protect against frost.  Typically done for potatoes, jerusalem artichokes, celery, leeks

eelworm : tiny transparent worms which adversly affect yields and/or quality of several plant varieties, notably potato, onion, chyrsanthemum.   Do not plant affected varieties for 6 years.  Being acid-loving, an effective control is to lime heavily and grow brassicas for a year or two.

etiolated : Adj : Of plants, developed without chlorophyll by being deprived of light; "etiolated celery".  Blanched.

eye : a bud or initial shoot on a tuber or rhizome

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F1 hybrid : a plant which has been specially bred to promote a particular feature - perhaps size, colour, yield or cropping time.  Seed for F1 hybrids are usually much more expensive & seed saved from F1 hybrid plants cannot be relied upon to breed true to type.

flea beetle : an insect pest of several plants, notably radish, swede, turnip, cime di rapa, chinese cabbage and others.  Can be ameliorated by planting a sacrificial crop nearby (chinese red mustard).  Infestations can be somewhat controlled by waving sticky cardboard / wallpaper paste over the plants.

frame : see coldframe

french drain : a shallow un-piped surface drain, usually filled level with gravel.  Correct placement channels excessive surface water to open ditches or soakaways.

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galanthus : HP : snowdrop

garlic : HA : strongest flavoured member of the allium family grown from division of parent plant (cloves).  I have classed as an HA since it matures within one growing year rather than one calendar year.  Plant autumn/early-winter to harvest in July

garlic spray : a solution of garlic, soap and water which acts as a deterrent to several types of insects, e.g. aphid, thrip.  Much used by organic gardeners.  Recipe, repeat applications usually required

graft : a technique by which a cutting from one plant is joined onto root stock from another.  Commonly used on fruit trees to give a particular type of fruit on a tree of a smaller size or different growth characteristic

green manure : a crop grown principally to improve the soil (fertility and/or soil structure and/or reduce leaching of nutrients).  Traditionally dug-in at the appropriate stage but may also be composted and added later.  Examples : mustard, tares, fenugreek, clover, hungarian grazing rye, lupins, buckwheat, alfalfa, phacelia, field beans

gypsum : calcium sulphate (hydrated) : useful for amelioration of heavy clay conditions.  It is approved for organic use provided it is in its pure form.  Sold commercially as Clay Breaker.  Try an agricultural supplier for bulk supplies.  e.g. http://www.rootwise.co.uk  Builder's gypsum (plaster) is not hydrated.

Growing Medium: some times known as compost, but not to be confused with Compost see above

Types of Growing Medium used by Professional Growers:

Potting, Potting and Bedding: Use to grow on seedlings
Nursery Stock - The next compost used after Potting and bedding for 2-3 year old plants
Blocking: used for larger plant module trays
Seed Sowing Modular: Not to be confused the Modular Seed Compost. SSM contains Vermiculite, and the other doesnt. Both are fine composts of low nutrient levels for germinating seeds in.
Tree and Shrub Compost

Growing Media are usually based on either Peat, or Peat free substitutes, such as Loam (soil) or Cocoa Fibre

Various Specialist Growing Media are also used, the names being self explanatory) such as:

Alpine and Herbaceous
Fruit Mixs such as Strawberry Mix, Raspberry Mix,
Mushroom Casing
Ericaceous (high acidity)
Primula And Pansy
Cactus (large chunks, lots of bark)
Aquatic (large percentage of coarse grit to stop it washing away)
Rose
Bulb Fibre (chunky loose medium with no nutrients to grow bulbs in)
African Violet

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HA : an hardy annual

half-hardy : a plant which is not tolerant of frosts / winter conditions in the area in which it is being grown.  See also hardy and tender.

hardening off : the process of acclimatising plants to outdoor conditions

hardy : a plant which is tolerant of frosts / winter conditions in the area in which it is being grown.  See also half-hardy and tender.

haulm : stem(s) of a plant especially potato, tomato, bean, pea

HB : an hardy biennial

heel-in: to plant something temporarily into soil - often a trench - to await planting into final position.  Basically to keep the roots moist.

HHA : an half-hardy annual

HHB : an half-hardy biennial (one which would need to be kept in a greenhouse, coldframe or shed over winter)

HP : an hardy perennial

HHP : an half-hardy perennial (one which would need to be kept in a greenhouse, coldframe or shed over winter)

hybrid : A variety of plant which has been selectively bred by a nursery or specialist breeder.  See also F1 hybrid

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ilex : Holly : any of several hardy evergreen trees / shrubs

imperial measurements, areal :
perch = 30.25 square yards (equivalent to a rectangle 1 rod wide by 1 rod long)
rood = 1210 square yards (equivalent to a rectangle 1 rod wide by 1 furlong long)
acre = 4840 square yards (equivalent to a rectangle 1 chain wide by 1 furlong long) - one acre equals 0.4047 hectare

imperial measurements, linear : rod = 5.5 yards, chain = 22 yards, furlong = 220 yards.  One yard equals 0.9144 metre.

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jerusalem artichoke : Not an artichoke but a type of sunflower grown for its edible tuberous roots.

juniper : any one of several coniferous trees / shrubs.

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kumara : See sweet potato

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lady's finger : see okra

legume : mostly HHA or HA : a family of plants which includes pea, bean and tares.  Notable in their ability to actually improve soil fertility by fixing atmospheric nitrogen into usable nitrates.  This is performed by bacteria present in nodules on the roots

lemon grass : TP : any one of approx 50 sub-tropical bunching grasses grown for their flavour.  Popular in eastern cuisines (notably Thai).  Can be grown from seed or by division of clumps

lime, horticultural or agricultural : ground limestone or chalk (primarily calcium carbonate).  Used to increase soil pH (decrease acidity).

lime, hydraulic : Also known as water lime.  This is produced from limestone containing clay and other impurities which enables it to set without exposure to air (unlike fat lime products). It will set underwater. Not for horticultural use.

lime, slaked or hydrated : ground limestone which has been fired in a kiln to produce higher alkalinity (primarily calcium hydroxide).  Builder's lime.  Should generally be avoided for horticultural purposes since it could be harmful to soil fauna, (notably earthworms).  Used to sweeten water in static tanks for the same reason.

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manure : animal or poultry excrement is described as manure when it is used for horticultural or agricultural purposes.  Typically combined with straw and aged aerobically before use.  Usually from cow or horse but most non-carnivorous sources are used to some extent.  Suitability for a given purpose depends hugely on the source and age.  If in doubt, manure can be added to the compost heap. how to choose, how to dig in, crops not liking manure, stacking fresh manure

mildew : any one of a family of fungal diseases, e.g. see powdery mildew.  Attacks are more frequent when soil is dry.

mound : See earthing up

mulch : sheeting and/or organic material which is laid on top of the soil around, or even over, the eventual crop.  Acts to suppress weeds and conserve moisture.  A mulch of organic material will also enrich the soil from increased earthworm activity.

mulch, floating : a layer of protective sheeting / fleece positioned above plants primarily to deter flying insects, birds, etc

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nitrogen robbery : a problem which occurs when unrotted or partially-rotted woody or cellulose material is incorporated into the soil.  Because the carbon:nitrogen is so high, nitrogen will be absorbed from the surrounding soil thus depriving plants of usable nutrients.

NPK : an abbreviation for nitrogen, phosphate, and potassium, the three principle plant foods