1. A term used to describe the position of governments, political parties, business groups, interest groups, or individuals who reject propositions on which a scientific or scholarly consensus exists: Some firms that are only interested in profits practice denialism when their interests and earnings are in danger of climate change, for example.
2. Choosing to contradict a reality as a way to avoid a disturbing actuality based on an empirically verifiable truth: AIDS denialism describes the disaffirmation of the facts which have been proven by relying on observation and tests.
3. An essentially irrational action that withholds validation of an historical experience or event: There are some people who still today use the term Holocaust denialism to present their views by refusing to admit that this part of the past ever occurred.

Apparently the term denialism is a neologism created by Michael Specter (a staff writer at The New Yorker since 1998) for his book, Denialism: How Irrational Thinking Hinders Scientific Progress, Harms the Planet, and Threatens Our Lives, published in 2009; Penguin Press; New York; in which Specter reveals that Americans have come to mistrust institutions and especially the institution of science more today than ever before.