Understatement is a figure of speech in which a writer or speaker deliberately makes a situation seem less important or serious than it is. As such, understatement can be used to reflect modesty, sarcasm, derogatory or complimentary tone.
For example: "He is a little on the old side" - describing a very old person.
The opposite of understatement is hyperbole. A hyperbole is an extreme exaggeration. These statements are not literally true, but people make them to sound impressive or to emphasize something.
For example: I nearly died laughing. I tried a thousand times.
Epithet (Greek - "addition") is a stylistic device emphasizing some quality of a person, thing, idea or phenomenon. Its function is to reveal the evaluating subjective attitude of the writer towards the thing described.
For example: It was opened by a small barrel of a woman, her fat arms shiny with suds.
He was a bald, vast-bearded man with a distinctly saturnine cast to his face; a Jeremiah.
An oxymoron (plural oxymora or oxymorons) (from Greek "sharp dull") is a figure of speech that combines contradictory terms. Oxymora appear in a variety of contexts, including inadvertent errors such as ground pilot and literary oxymorons crafted to reveal a paradox.
For example: Bitter sweet, dark light, mad wisdom etc.
http://grammar.about.com/od/tz/g/understateterm.htm
https://sites.google.com/site/afigurativelanguagewebquest/task/day-8--hyperbole
http://estylistics.blogspot.com/2012/05/epithet-as-stylistic-device.html