Denial, in ordinary English usage, is asserting that a statement or allegation is not true (which might be accurate or inaccurate). It may also mean the refusal of a request, but this article covers denial of true factual claims.
In psychology, denialism is a person's choice to deny reality as a way to avoid a psychologically uncomfortable truth.
In psychoanalytic theory (which has been criticized as unscientific and factually unfounded), denial is a defense mechanism in which a person is faced with a fact that is too uncomfortable to accept and rejects it instead, insisting that it is not true despite what may be overwhelming evidence. The concept of denial is important in twelve-step programs where the abandonment or reversal of denial that substance dependence is problematic forms the basis of the first, fourth, fifth, eighth and tenth steps.
People who are exhibiting symptoms of a serious medical condition sometimes deny or ignore those symptoms because the idea of having a serious health problem is uncomfortable or disturbing. The American Heart Association cites denial as a principal reason that treatment of a heart attack is delayed. Because the symptoms are so varied, and often have other potential explanations, the opportunity exists for the patient to deny the emergency, often with fatal consequences. It is common for patients to delay recommended mammograms or other tests because of a fear of cancer, even on average this worsens the long-term medical outcome.