A definition is a verbal description of the concept. Follow the principles in ISO 704 - Terminology work - Principles and methods for the style of writing definitions. Each entry must have one, and only one, definition.

A definition can contain a term which is defined elsewhere in the standard. Such terms are called "entailed terms" and they must be enclosed in the entailedTerm element so that a link will be generated to point to the location where the definition of that term can be found. A definition can also contain lists, tables, other special markup, and certain symbols (those that can be expressed in a markup language such as MathML or via characters accessible from the keyboard).

Definitions are only allowed at the langSet level. This supports multilingual entries without needing definitions at multiple levels, and it facilitates translation of ISO terminology.