A cactus (plural: cacti) is any member of the spine plant family Cactaceae, native to the Americas. They are often used as ornamental plants, but some are also crop plants.

Cacti are distinctive and unusual plants, which are adapted to extremely arid and hot environments, showing a wide range of anatomical and physiological features which conserve water. Their stems have expanded into green succulent structures containing the chlorophyll necessary for life and growth, while the leaves have become the spines for which cacti are so well known.