An example of an opuntia is the prickly pear cactus.
opuntia definition by Webster's New World
Origin: Modern Latin ; from Classical Latin (herba) Opuntia, (plant) of Opus, city in Locris
opuntia definition by American Heritage Dictionary
Origin: Latin (herba) Opūntia, Opuntian (herb), feminine of Opūntius, of Opus, an ancient town of east-central Greece, from Opūs, Opūnt-, Opus, from Greek Opous, Opount-.