In Chomskyan syntax, pro (also called little pro, to distinguish it from PRO in speech) is a null pronoun-like noun phrase that differs from PRO in that it occurs in a position where it can get Case. Little pro is often said to be licensed by rich agreement.
Spanish viene '(he) comes' is analyzed as pro viene.
pro in (i)b (which only exists in underlying structure) is interpreted as the subject.
(i) a Piero conosce bene Maria Piero knows Maria well b pro Conosce bene Maria He knows Maria well
Pro must be licensed. A pro subject is licensed by (morphological) agreement features. It is claimed that pro may occur in object positions in Italian.
pro was introduced by Chomsky 1981, as an abbreviation of pronoun.