When you plant a seed the first set of leaves are called the seed-leaf. Some plants will even put off a second kind of leaf after the seed-leaf, but then by the third set produce what is called the "true leaf" which is the representation of the adult leaf.
Most, however, go directly from the seed-leaf to the true leaf; it is the reference of identification of a seedling as to what kind of plant it is. You might hear someone say when asked "what is it?" that they tell you to wait until you see the 'true-leaf' to make a determination.
