Definition - A word or expression that stands for another word, phrase, clause, or sentence. Some types of pro-forms are:
(1) pronouns (substitute for nouns), e.g. "Did you see Mr. Jones' new pink Cadillac." — "Yes, I saw it."
(2) pro-verbs (substitute for verbs), e.g. "Why can't he do it?" — "It's not that he can't [do it], he just won't [do it]."
(3) pro-sentences (substitute for sentences), e.g. "Are you going?" — "Yes [I am going]."

Oxford English Dictionary -
The term's first citation is from 1964:
"We stipulate in the general theory of linguistic descriptions that the dictionary entry of every *pro-form (i.e. every form dominated by the constituent Pro) must contain the semantic marker (Selector)."
(Katz & Postal Integrated Theory Ling. Descr. iv. 83)

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