A cactus is a member of the plant family Cactaceae, within the order Caryophyllales. The word cactus is derived through Latin from the Ancient Greek κάκτος (kaktos), a name originally used for a spiny plant whose identity is not certain. The plural of cactus varies; the Latin cacti, the English cactuses and the uninflected plural cactus are all used. With the exception of one species, cacti are native to the Americas, ranging from Patagonia in the south to parts of western Canada in the north.
Most cacti live in habitats which are subject to at least some degree of 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 reduce air flow close to the cactus and provide some shade, both of which help to prevent water loss. 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 multi-petaled.