A cotyledon is a significant part of the embryo within the seed of a plant, and is defined as "the embryonic leaf in seed-bearing plants, one or more of which are the first to appear from a germinating seed". Cotyledons help supply the nutrition a plant embryo needs to germinate and become established as a photosynthetic organism and may themselves be a source of nutritional reserves or may aid the embryo in metabolizing nutrition stored elsewhere in the seed.

The number of cotyledons present in a seed varies depending on the type of plant. Angiosperms (flowering plants) whose embryos have a single cotyledon are grouped as monocots, or monocotyledonous plants; most embryos with two cotyledons are grouped as eudicots, or eudicotyledonous plants. Gymnosperm seedlings also have cotyledons, and the number of cotyledons in the embryos of seeds of gymnosperms is highly variable, ranging from 8 to 20 or more.

In many monocots, the cotyledon acts as a special absorbing organ to mobilize the reserve materials and withdraw them from the endosperm. The cotyledon of grasses and many other monocotyledons is a highly modified leaf composed of a scutellum and a coleoptile. The scutellum is a tissue within the seed that is specialized to absorb stored food from the adjacent endosperm. The coleoptile is a protective cap that covers the plumule (precursor to the stem and leaves of the plant) .