A cotyledon is defined as an embryonic leaf inside seed-bearing plants. One or more of the cotyledons will be the first leaves to appear in the germinating plant (this is what we call the plant when it first begins to emerge from the ground).
Cotyledons can be either monocotyledons or dicotyledons. Monocotyledons contain only one embryonic leaf inside the seed-bearing plant ("mono" = one), whereas dicotyledons contain two ("di" = two).
Having a monocotyledon vs. a dicotyledon is one distinction between monocots and dicots, which are two kinds of flowering plants. Monocots and dicots can further be distinguished by their vascular bundles, number of petals, leaf veins, and root systems. This hyperlinked chart outlines the characteristics for each plant part for a monocot verses for a dicot.