Etymology -From the Latin word cereus, meaning a wax taper (candle).
Earlier in cactus taxonomy, Cereus is a name that has been applied to nearly all known cactus species that were ribbed, columnar plants. Many of these plants have since been moved out into separate genera. Consequently, the 30 or so plants that remain in the Cereus group are largely plants that have not been moved out of the genus rather than plants that have been included because they fit the description of Cereus. This inclusion-by-lack-of-exclusion makes for a very messy and unsatisfactory grouping.
The stereotypic Cereus is a large, tree-like columnar plant with 4 to 10 well-defined ribs, large areoles, and moderate, stout spines. The flowers are large and white with floral tubes that are smooth or have smooth scales; they are nocturnal and fragrant. Some of the species that fit nicely into this description are C. hexagonus, C. hildmannianus, C. jamacaru, C. lanosus, and C. repandus among others. Exceptions to this are the sprawling, thin, climbing species like C. albicaulis, C. spegazzinii, and C. kroenleinii. A shrubby, thin-stemmed species, C. insularis resembles the Genus Cleistocactus while C. mortensenii bears a pseudocephalium.
Needless to say, the Cereus genus requires much more study and will likely lose more species to other genera as a result.