Succulence is an adaptive strategy that allows plants to remain active during seasonal water shortage. The term was first used formally by Johann (Jean) Bauhin in 1619 to refer to plants with thick, juicy leaves. Its subsequent use and selected definitions are critically discussed, including concepts such as utilizable water, caudiciforms and pachycauls, and root succulence. A unified definition of succulence considers aspects of morphology and anatomy, ecology, and physiology. Stem succulence and the “cactus life form” are used to illustrate the parallel evolution of functional adaptations in morphology, and to contrast the obvious external similarities with the widely variable internal architecture, including the participation of different stem tissues in water storage.