Well, there's not much doubt it's a species of Cereus (that is, the true Cereus genus). But the correct identification of this commonly grown large Cereus has always been a matter of debate.
For many years it was known as C. peruvianus, but that name has turned out to be wrongly applied to this plant. More recently it was identified as C. uruguayanus, which, as the name suggests, comes from more southern parts of South America. That is now being treated as a subspecies of C. hildmannianus (see Anderson, The Cactus Family) - so its name becomes Cereus hildmannianus subsp. uruguayanus - rather a mouthfull!
