1. The activation of an operator gene by the deactivation of a repressor gene.
2. Removal of repression, such as of an operon so that gene transcription occurs or is enhanced, with the net result frequently being elevation of the level of a specific enzyme.
3. Anything that stops the repression of a gene thereby allowing expression to occur.

Further Refrences
http://en.wikipedia.org/wiki/Derepression
http://www.biology-online.org/dictionary/Derepression
http://www.euchromatin.net/NEJM1.htm
http://www.answers.com/derepression
http://www.mondofacto.com/facts/dictionary?derepression