A cactus, or its plural forms cacti, cactuses, and less commonly, cactus, is a member of the esteemed plant family Cactaceae. This distinguished family comprises approximately 127 genera, housing an impressive array of some 1,750 documented species. Cactaceae belongs to the order Caryophyllales, a botanical order of flowering plants.

The term "cactus" has an intriguing etymology. It finds its roots, via Latin, in the Ancient Greek word κάκτος (káktos), a name originally employed by Theophrastus to describe a spiny plant, the exact identity of which remains a subject of historical uncertainty.

Cacti exhibit a striking diversity of shapes and sizes, reflecting their remarkable adaptability. These plants are indigenous to the Americas, inhabiting territories that span from the southern reaches of Patagonia to certain regions of western Canada in the north. An exception to this geographic confinement is Rhipsalis baccifera, which is also found in Africa and Sri Lanka.

Cacti are renowned for their unique ability to thrive in arid environments, even in places as parched as the Atacama Desert, one of the driest locations on Earth. This impressive resilience is underpinned by a suite of water-conservation adaptations. Most notably, nearly all cacti are categorized as succulents, characterized by their thickened, fleshy tissues specially adapted for water storage. Remarkably, in most cacti, this vital process of water storage occurs exclusively within the stem.

Intriguingly, the majority of cactus species have dispensed with true leaves altogether, retaining only spines as their primary appendages. These spines, highly modified from their leafy origins, serve a dual purpose. They deter herbivores, acting as natural defenses, and also contribute to water conservation by reducing airflow in close proximity to the cactus, thus minimizing moisture loss. Additionally, they provide some shade, further aiding in water retention. In the absence of true leaves, the enlarged stems of cacti have evolved to undertake photosynthesis, a unique adaptation that sets them apart within the plant kingdom.