A cactus (plural: cacti, cactuses, or cactus) is a member of the plant family Cactaceae, within the order Caryophyllales. The word "cactus" derives, through Latin, from the Ancient Greek κάκτος (kaktos), a name originally used for a spiny plant whose identity is not certain. Cacti are native to the Americas, ranging from Patagonia in the south to parts of western Canada in the north—except for Rhipsalis baccifera, which also grows in Africa and Sri Lanka.
Most cacti live in habitats subject to at least some drought. Many live in extremely dry environments, even being found in the Atacama Desert, one of the driest places on earth. Cacti show many adaptations to conserve water. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves. As well as defending against herbivores, spines help prevent water loss by reducing air flow close to the cactus and providing some shade. Cactus spines are produced from specialized structures called areoles, a kind of highly reduced branch. Areoles are an identifying feature of cacti. As well as spines, areoles give rise to flowers, which are usually tubular and multipetaled.
In the absence of leaves, enlarged stems carry out photosynthesis. Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place. Cactus stems also store water, and are often ribbed or fluted, which allows them to expand and contract easily. Cacti occur in a wide range of shapes and sizes. The tallest free-standing cactus is Pachycereus pringlei, with a maximum recorded height of 19.2 m (63 ft), and the smallest is Blossfeldia liliputiana, only about 1 cm (0.4 in) in diameter at maturity. The smaller cacti usually have globe-shaped stems, combining the highest possible volume with the lowest possible surface area. Many cacti have short growing seasons and long dormancies, and are able to react quickly to any rainfall, helped by an extensive but relatively shallow root system. A fully grown saguaro (Carnegiea gigantea) is said to be able to absorb as much as 200 US gallons (760 l; 170 imp gal) of water during a rainstorm.