Definition of pro noun from the Oxford Advanced Learner's Dictionary. a person who works as a professional, especially in a sport. He handled the situation like an old pro (= somebody who has a lot of experience). She’s a real pro.
1. a prefix, having anti- as its opposite, used to form adjectives that have the general sense “favoring” the group, interests, course of action, etc., denoted by the headword: pro-choice; pro-American; prowar.
pro- comes from Greek, and has the meaning "before, beforehand, in front of'': proboscis; prophylactic; prothesis. pro- is also attached to a word and means "primitive or early form'': prodrug; prosimian.
