Real Dictionary

oxymoron
countable noun: If you describe a phrase as an oxymoron, you mean that what it refers to combines two opposite qualities or ideas and therefore seems impossible.

oxymoron in American English
a figure of speech in which opposite or contradictory ideas or terms are combined (Ex.: thunderous silence, sweet sorrow)
noun: a figure of speech by which a locution produces an incongruous, seemingly self-contradictory effect, as in “ cruel kindness” or “to make haste slowly”

oxymoron in British English
noun: an epigrammatic effect, by which contradictory terms are used in conjunction