An oxymoron is the pairing of two or more words to create a meaning that is contradictory or seems to be contradictory. A couple of examples of oxymorons that seem contradictory include objective opinion, speed bump, and jumbo shrimp. (For what is billed as the “largest list of oxymorons collected online,” please visit oxymoronlist.com.) These, however, are logical pairings of words, and since they’re logical, they are reasonable ways to communicate. In fact, I wouldn’t mind a few jumbo shrimp in a scampi sauce now.
Some oxymorons don’t just seem to be contradictory; they are contradictory, which renders them ineffective as communication tools–that would be illogical communication, to coin my own oxymoron. To me, the always-popular definite possibility falls into that category since there is no logical reason to pair the certainty associated with the word definite and the uncertainty associated with the word possibility. How can something be certain and uncertain at the same time? We would never pair the equally contradictory possibly definite in any instance.
I understand that the intention is to express a smaller amount of uncertainty, such as a house shopper might say This house is a definite possibility about a house that nearly meets all of the family’s needs and is worth further consideration. A better way to express it, though, would be to say This house is a distinct possibility since it expresses the intended thought without the obvious contradiction.
–Paul
Paul’s book–Literally, the Best Language Book Ever and Sherry’s Grammar List

