A cactus (plural: cacti, cactuses or cactus) is a member of the plant family Cactaceae, within the order Caryophyllales. Their distinctive appearance is a result of adaptations to conserve water in dry and/or hot environments. In most species, the stem has evolved to become photosynthetic and succulent, while the leaves have evolved into spines. Many species are used for ornamental plants, and some are also grown for fodder, forage, fruits, cochineal, and other uses.

Cacti come in a wide range of shapes and sizes. The tallest is Pachycereus pringlei, with a maximum recorded height of 19.2 m, and the smallest is Blossfeldia liliputiana, only about 1 cm in diameter at maturity. Cactus flowers are large, and like the spines arise from distinctive features called areoles.