What is an oxymoron?

An oxymoron (usual plural oxymorons, more rarely oxymora) is a rhetorical device that uses an ostensible self-contradiction to illustrate a rhetorical point or to reveal a paradox. More general meaning of "contradiction in terms" (not necessarily for rhetoric effect) is recorded by the OED for 1902.

The term is first recorded as latinized Greek oxymōrum, in Maurus Servius Honoratus (c. AD 400); it is derived from two Greek words meaning "sharp, keen, pointed" and "dull, stupid, foolish"; as it were, "sharp-dull", "keenly stupid", or "pointedly foolish". The word oxymoron is autological, i.e. it is itself an example of an oxymoron. The Greek compound word ὀξύμωρον oksýmōron, which would correspond to the Latin formation, does not seem to appear in any known Ancient Greek works prior to the formation of the Latin term.