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 can vary depending on the type of plant. For example, angiosperms (flowering plants) whose embryos have a single cotyledon are grouped as monocots, or monocotyledonous plants, while 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.

Cotyledons may be ephemeral, lasting only days after emergence, or persistent, enduring at least a year on the plant. The cotyledons contain (or in the case of gymnosperms and monocotyledons, have access to) the stored food reserves of the seed. As these reserves are used up, the cotyledons may turn green and begin photosynthesis, or may wither as the first true leaves take over food production for the seedling.

In summary, cotyledons are the first leaves produced by plants and are part of the embryo within the seed. They 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 can vary depending on the type of plant.