Botanists have reasonably accurate definitions for them, but basically they are perennial flowering plants that store water in some way or other. Trouble is, that according to strict botanical definitions, there are some plants that should be included in books on succulents, and some that shouldn't be present, but are. A lot of the Orchidaceae should be included but aren't. The only way to be sure that a plant is classed as a succulent is to consult botanical literature to see if it is in or out.

Cacti are the most obvious succulents, but there are many other plant families having species exhibiting succulence. Cacti are separated by succulent collectors from the 'Other Succulents' simply because there are a large number of people studying and collecting this family. The split is for convenience only.

There is a relatively new name for certain types of succulents, Pachycauls. These are mainly tree like plants with swollen stems. Trouble is the swollen stems are not always for water storage, but for food production. Again, some of the plants included in succulent books of this type are not succulents according to the true botanical definition of succulents.

The truth is that succulent collectors and interested botanists are attracted to these plants purely for their shape and form so any plant that looks like a known succulent is likely to be a candidate. Orchids have other organisations studying them, and no self respecting succulentophile would ever tread on any one else's toes.