Phylloclades and cladodes are flattened, photosynthetic shoots, which are modified branches. The two terms are used differently or interchangeably by different authors. Phyllocladus, a genus of conifer, is named after these structures. Phylloclades/cladodes have been identified in fossils dating from as early as the Permian.
By one definition, phylloclades are a subset of cladodes, those that greatly resemble or perform the function of leaves, as in Butcher's broom (Ruscus aculeatus) as well as Phyllanthus and some Asparagus species.
By an alternative definition, cladodes are distinguished by their limited growth and that they involve only one or two internodes. By this definition, some of the most leaf-like structures are cladodes, rather than phylloclades. By that definition, Phyllanthus has phylloclades, but Ruscus and Asparagus have cladodes.
Currently, only prickly pears are included in this genus of about 200 species distributed throughout most of the Americas. Chollas are now separated into the genus Cylindropuntia, which some still consider a subgenus of Opuntia. Austrocylindropuntia, Corynopuntia and Micropuntia are also often included in the present genus, but like Cylindropuntia they seem rather well distinct. Brasiliopuntia and Miqueliopuntia are closer relatives of Opuntia.
The most commonly culinary species is the Indian Fig Opuntia (O. ficus-indica). Most culinary uses of the term "prickly pear" refer to this species. Prickly pears are also known as "tuna", "nopal" or nopales, from the Nahuatl word nōpalli for the pads, or nostle, from the Nahuatl word nōchtli for the fruit; or paddle cactus.
The genus is named for the Ancient Greek city of Opus where, according to Theophrastus, an edible plant grew which could be propagated by rooting its leaves.