In human behavior, denialism is exhibited by individuals choosing to deny reality as a way to avoid dealing with an uncomfortable truth. Author Paul O'Shea remarks, "[It] is the refusal to accept an empirically verifiable reality. It is an essentially irrational action that withholds validation of a historical experience or event". Author Michael Specter defined group denialism as “when an entire segment of society, often struggling with the trauma of change, turns away from reality in favor of a more comfortable lie.”
In science, denialism has been defined as the rejection of basic concepts that are undisputed and well-supported parts of the scientific consensus on a topic in favor of ideas that are both radical and controversial. It has been proposed that the various forms of denialism have the common feature of the rejection of overwhelming evidence and the generation of a controversy through attempts to deny that a consensus exists. A common example is Young Earth creationism and its dispute with the evolutionary theory.
One who engages in denialism is a denialist.
Recent references have been made to the term denihilist as well, which isn't bad, either, but more humorous than apropos to this question.
Synonyms: skepticism, negativity, cynicism, pessimism; disbelief, unbelief, agnosticism, atheism.