仙人掌是石竹目仙人掌科（学名：Cactaceae）的植物总称，别名为仙巴掌、仙人扇、霸王树。仙人掌具有相当丰富的形状和大小，并拥有优良的保水适应力，多数生长于沙漠及半沙漠等干燥少雨环境，甚至在地球上最干旱的地区之一阿塔卡马沙漠也得以发现。仙人掌为多肉植物的一类，目前仙人掌科的植物有127属共1896种。

仙人掌用途广泛，可作观赏植物、饲用或饲料，以及其他食物来源（特别是它们的果实​​）。 胭脂虫是生活在某些仙人掌中一种昆虫的产物。

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 by Theophrastus for a spiny plant whose identity is not certain. Cacti occur in a wide range of shapes and sizes. 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. Almost all cacti are succulents, meaning they have thickened, fleshy parts adapted to store water. Unlike many other succulents, the stem is the only part of most cacti where this vital process takes place. 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. In the absence of leaves, enlarged stems carry out photosynthesis. 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.