Oxymoron is a figure of speech in which two contradictory words or phrases are used together to make a statement which seems to be absurd or self-contradictory – for example, the observation (made by King Henri IV of France about James VI and I) that "He is the wisest fool in Christendom". Other examples are "Faith unfaithful kept him falsely true" (Alfred, Lord Tennyson (1809-1892), Lancelot and Elaine, from Idylls of the King); "I am a deeply superficial person" (Andy Warhol); and "I am busy doing nothing."
Oxymoron and paradox are clearly related (since someone who uses oxymoron may be said to express themselves in a paradoxical way). However, the two should not be confused. Oxymoron is a figure of speech or rhetorical device, i.e., it enables us to say in a paradoxical (and therefore striking way) something that can be said in an unparadoxical (but less striking) way. Hence a statement involving oxymoron can always be translated without loss of meaning into a statement in which there is no oxymoron. For example, Henri IV’s observation about King James could be paraphrased as ‘Although he is well-educated and learned, he makes bad and foolish decisions’. By contrast, a paradox is not a figure of speech or rhetorical device, but (a statement about) a state of affairs which is absurd, self-contradictory, or for some other reason impossible. A (genuine) paradox cannot be eliminated by simply reformulating the statement in which it is made.
Etymological note: The word oxymoron comes, through Latin, from the Greek ὀξύμωρον (oxumōron), used of a witty remark which is all the more pointed for seeming to be absurd. The Greek word is a compound of two adjectives, ὀξύς (oxus) meaning 'sharp', and μωρός (mōros) meaning, 'dull, sluggish, stupid'.
See also paradox.