pot sizes

Simple question really, and probably incredibly dumb!  When people refer to 3 inch pots, do they mean the diameter of the top of the pot, or the depth?   

When I do I refer to Diameter - but dunno about other people!

I expect 3" diameter pots are also about 3" deep. Problem arises with larger sizes. There are "Pans" - which are shallow relative to diameter - and things like Rose Pots which are deep relative to the diameter.

And then some pots are sized in Litres, rather than Diameter.

All very confusing I'm afraid.

What are you wanting to grow / plant ?

Coolio, thanks!  Just wanted to make sure Im not using too big pots for potting on my tom and chilli seedlings - Ive read that if you give them too much space it can be bad (forget the reason!).