Oxymoron

From Wikipedia: An oxymoron is a figure of speech that combines two normally contradictory terms. Oxymoron is a Greek term derived from oxy ("sharp") and moros ("dull").

In popular usage, the term oxymoron is sometimes used more loosely, in the sense of a simple contradiction in terms. Often, it is then applied to expressions which, unlike real oxymora, are used in full earnest and without any sense of paradox by many speakers in everyday language.

• "bitter sweet"
• A rhetorical figure in which incongruous or contradictory terms are combined, as in a "deafening silence" and a "mournful optimist". (freedictionary.com)
• "organised chaos" (from tv programme)