Oxymoron
The juxtaposition of two contradictory ideas is oxymoron, from
the Greek words for "sharp" and "foolish." Everybody's favorite
examples are facetious ("military intelligence"), but poets can
often use oxymoron for striking effects (such as Milton's
"darkness visible").
From the Guide to Literary Terms by Jack Lynch.
Please send comments to Jack Lynch.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.