Noun

1. Any member of the family Cactaceae, a family of flowering New World succulent plants suited to a hot, semi-desert climate.
2. Any succulent plant with a thick fleshy stem bearing spines but no leaves, including euphorbs.

Usage notes

The most precise definition of cactus includes only plants from the New World (the Americas) belonging to the family Cactaceae. Only one species of cactus is native to the Old World, namely the genus Pereskea is a shrubby leafy cactus that grows in western Africa. However, many African species of the family Euphorbiaceae grow in forms and shapes that resemble large species of Cactaceae. These forms are colloquially referred to as “cactus”. In general, then, any usage of the term “cactus” to plants from the Old World refers to plants in the Euphorbiaceae.