"Denial is a defense mechanism in which a person is faced with a fact that is painful to accept rejects it instead, insisting that it is not true despite what may be overwhelming evidence. The subject may deny the reality of the unpleasant fact altogether (simple denial), admit the fact but deny its seriousness (minimisation) or admit both the fact and seriousness but deny responsibility (transference). The concept of denial is particularly important to the study of addiction."

http://en.wikipedia.org/wiki/Denial