Contrast Clues are words or phrases that provide the opposite meaning of an unknown word, thus hinting to the true meaning of that word.  When this type of clue is used, the reader realizes that the word must mean the opposite of the contrast clue used by the writer.
 

Examples:
 

Rosie and Tyrone thought they were being discreet when they saw each other outside of the workplace; however the entire office knew that they were secretly dating.

For now our weekend trip is tentative, unless of course, the airline calls to confirm our flight.