Wiktionary
contradiction in terms
n. A phrase or expression in which the component words contradict one another, often unintentionally, or are claimed to do so when seen from a particular point of view.
WordNet
contradiction in terms
n. (logic) a statement that is necessarily false; "the statement `he is brave and he is not brave' is a contradiction" [syn: contradiction]
Usage examples of "contradiction in terms".
SCENT agents were supposed to remain inconspicuous, and a softhearted medieval bourgeois was a contradiction in terms.
So a proprietary, closed, secret operating system is a contradiction in terms.