Scientific classification:
Superdomain: Neomura
Domain: Eukaryota
(unranked): Archaeplastida
Kingdom: Plantae
Subkingdom: Tracheobionta
Superdivision: Spermatophyta
Division: Magnoliophyta
Class: Magnoliopsida
Subclass: Caryophyllidae
Order: Caryophyllales
Family: Cactaceae
Subfamily: Cactoideae
Tribe: Cereeae
Genus: Cereus
Species: C. forbesii

From my collection

via Flickr https://flic.kr/p/2itfJDj