A plastid is any of a family of membrane-bound organelles unique to plant cells. Several types of plastids with widely different functions exist, although only one type is found in each cell. The different types are related because they all differentiate from a common precursor organelle called a proplastid.

Chloroplasts (containing chlorophyll) are the most widely known form of plastid. Other types are chromoplasts (containing other pigments) and leucoplasts (unpigmented).