An oxymoron (plural "oxymora", also "oxymorons") is a short phrase that appears self-contradictory. The oxymora belong to the genus contradictia, i.e. they are a proper subset of the expressions called contradiction in terms. What distinguishes oxymora from other contradictia is that they are used intentionally, for rhetorical efffect, and the contradiction is only apparent.

The most common form of oxymoron involves an adjective-noun combination. For example, the following line from Tennyson's Idylls of the King contains two oxymora:

And faith unfaithful kept him falsly true

The term "deafening silence" is an oxymoron if the user is aware of the contradiction and is using it intentionally; otherwise, it is just a cliche.

Humorous, sardonic or sarcastic commentary is the most frequent context for an oxymoron:

I do here make humbly bold to present them with a short account of themselves... -- Jonathan Swift
The bookful blockhead, ignorantly read, / With loads of learned lumber in his head... -- Alexander Pope
He was now sufficiently composed to order a funeral of modest magnificence... -- Samuel Johnson

There is a class of expressions that is often labeled oxymora but is actually quite different. An oxymoron is the deliberate use of two apparently contradictory words that turn out not to be contradictory in the context. The "non-oxymorons" are expressions that do not appear to be contradictory, but contain a contradiction if you happen to be in on the joke. (Thus, you can think of them as crypto-contradictions.) Expressions such as "old news", "extensive briefings", "random order", "detailed summary" are contradictory only if one takes into account a meaning of a word different from the meaning intended. "Jumbo shrimp", for example, is a contradiction in terms only if shrimp doesn't refer to a crustacean. This shift in meaning can be used for humorous or satirical purposes: "government initiative", "military intelligence", "neutral point of view", "female rationality", "male sensitivity", "Visual Basic Programmer";, "corporate ethics"; etc.