oxymoron

The Greeks exhaustively classified the elements of rhetoric, or effective speech and writing, and gave the name oxymoron, literally "pointed foolishness," to the deliberate juxtaposing of seemingly contradictory words. The roots of oxymoron - oxys, meaning "sharp" or "keen," and moros, meaning "foolish" - are nearly antonyms themselves, making oxymoron nicely self-descriptive. Oxymoron originally applied to a meaningful paradox condensed into a couple of words, as in "precious bane," "lonely crowd," or "sweet sorrow." Today, however, oxymoron can also refer to unintentional contradictions, like "a plastic glass."