The Cactoideae are the largest subfamily of the cactus family, Cactaceae, and are widely distributed throughout the Americas. Cactaceae is the 5th most endangered plant or animal family evaluated globally by the International Union for Conservation of Nature. Around 80% of cactus species belong to this subfamily. The genera of the Cactoideae are characterized by microscopic foliage leaves. All photosynthesis occurs in shoot cortex cells covered by a persistent epidermis and stomata. Another important characteristic of this subfamily is ribbed stems, which enable the inner cortex to expand radially without breaking the shoot surface to absorb large quantities of water.

A cactus is a member of the plant family Cactaceae, a family comprising about 127 genera with some 1,750 known species of the order Caryophyllales. The word cactus derives, through Latin, from the Ancient Greek word κάκτος (káktos), a name originally used by Theophrastus for a spiny plant whose identity is now not certain. Cacti occur in a wide range of shapes and sizes. They are native to the Americas, ranging from Patagonia in the south to parts of western Canada in the north, with the exception of Rhipsalis baccifera, which is also found in Africa and Sri Lanka. Cacti are adapted to live in very dry environments, including the Atacama Desert, one of the driest places on Earth. Because of this, cacti show many adaptations to conserve water. For example, 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 true leaves, cacti's enlarged stems carry out photosynthesis.