Pro-form

A pro-form is a type of function word or expression that stands in for (expresses the same content as) another word, phrase, clause or sentence where the meaning is recoverable from the context. They are used either to avoid repetitive expressions or in quantification (limiting the variables of a proposition).

Pro-forms are divided into several categories, according to which part of speech they substitute:

An interrogative pro-form is a pro-form that denotes the (unknown) item in question and may itself fall into any of the above categories.