PRO (also called big PRO, and different from 'little pro') stands for the word pronoun, and it's a term used in linguistics, specifically in syntax-semantics, to refer to an empty category:
Ex: I persuaded John [PRO] to read Al Gore's latest book. The noun John has two functions, one semantic and one syntactic. John is the semantic subject of infinitive to read, and the syntactic object of the main verb persuaded. In G&B (Government & Binding Theory, Chomsky) any given constituent cannot have more than one syntactic role; therefore, PRO is added to support the syntax-semantics interface. PRO (pronoun) refers back to John.
See also here PRO (linguistics - Wikipedia, the free encyclopedia) and here http://www.google.com/search?hl=en&n...ro+linguistics.