Often these are called the seed leaves. They are the first leaves that appear when a seed germinates and are usually rounder and fleshier than the true leaves. The growth stages of plants are often refered to as cotyledon, first true leaves, second true leaves....... flower bud, flower, seed set ... and so on.
Not all plants have seed leaves above ground. Peas, broad beans and runner beans all leave the cotyledens below ground in the seed husk so the first leaves you see are true leaves.
In the GROWING STICKIES section there is a Glossary of Terms which will tell you what most gardening words mean. Unfortunately cotyleden in not in there?