In semantics, a modifier is said to be restrictive (or defining) if it restricts the reference of its head. For example, in "the red car is fancier than the blue one", red and blue are restrictive, because they restrict which cars car and one are referring to. ("The car is fancier than the one" would make little sense.) By contrast, in "John's beautiful mother", beautiful is non-restrictive; "John's mother" identifies her sufficiently, whereas "beautiful" only serves to add more information.

Restrictive modifiers are also called defining, identifying, essential, or necessary; non-restrictive ones are also called non-defining, non-identifying, descriptive, or unnecessary (though this last term can be misleading). In certain cases, generally when restrictiveness is marked syntactically through the lack of commas, restrictive modifiers are called integrated and non-restrictive ones are called non-integrated or supplementary.