A compendium is a short but comprehensive compilation of a body of knowledge. A compendium usually contains principal heads, or general principles, of a larger work or system.

Limits

The reference to a larger system is traditionally used meaning something that can not be easily defined. Practically speaking there is a scope. Everything can be broken down into smaller components and everything can be a larger system of something else. There is a bottom level, however, and that level consists of simple associations called definitions.

A good definition is an association of two things: A word and its meaning. The larger system is the word. The components are the sound and its association. In such a way a definition can be considered the final limit of a compendium.

Encyclopedias

Traditional

A traditional encyclopedia can be referred to a compendium of human knowledge.

Wikipedia

The free content, collaboratively edited encyclopedia, Wikipedia, can change the traditional meaning of the word compendium because of its ability to use hyperlinks to break things into their components and because of its ability to maintain a solid order of levels (based on the cumulative entries of the editors).

Etymology

The word compendium comes from the Latin word compendere, meaning "to weigh together".