The structure in angiosperms and gymnosperms that develops from the ovule after fertilization. Occasionally seeds may develop without fertilization taking place (see apomixis). The seed contains the embryo and nutritive tissue, either as endosperm or food stored in the cotyledons. Angiosperm seeds are contained within a fruit that develops from the ovary wall. Gymnosperm seeds lack an enclosing fruit and are thus termed naked. The seed is covered by a protective layer, the testa. During development of the testa the seed dries out and enters a resting phase (dormancy) until conditions are suitable for germination.

Annual plants survive the winter or dry season as seeds. The evolution of the seed habit enabled plants to colonize the land, since seed plants do not depend on water for fertilization (unlike the lower plants).