The Standard uses the phrase, “user-defined type,” but it is not clear what it is intended to mean. For example, 16.4.5.2.1 [namespace.std] paragraph 1 says,

A program may add a template specialization for any standard library template to namespace std only if the declaration depends on a user-defined type...

Are types defined in the Standard library “user-defined?”

9.2.9.3 [dcl.type.simple] paragraph 2 says,

The auto specifier is a placeholder for a type to be deduced (9.2.9.7 [dcl.spec.auto]). The other simple-type-specifiers specify either a previously-declared user-defined type or one of the fundamental types (6.8.2 [basic.fundamental]).

implying that all non-fundamental types are “user-defined.”

A definition is needed, as well as a survey of uses of the term to ensure consistency with the definition.