A 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. Prickly pears typically grow with flat, rounded cladodes (also called platyclades) armed with two kinds of spines; large, smooth, fixed spines and small, hairlike prickles called glochids, that easily penetrate skin and detach from the plant. Many types of prickly pears grow into dense, tangled structures. 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.
According to Wikipedia, when Carl Linnaeus published Species Plantarum in 1753 – the starting point for modern botanical nomenclature – he placed all the species of cactus known to him in one genus, Cactus. In 1754, the Scottish botanist Philip Miller divided cacti into several genera, including Opuntia. He distinguished the genus largely on the form of its flowers and fruits.