Denialism is a mechanism which people or organizations employ in order to avoid inconvenient or uncomfortable truths.
It refers to the choice of "denying reality" in order to champion a claim or theory.
One of the most well-known examples of denialism concerns the assertion that the extermination of Jews (see Holocaust denial) during the second world war never in fact happened.
Another example is the ex-President of South Africa, Thabo Mbeki, who argued against the scientific consensus that HIV caused AIDS. This led to policies preventing thousands of HIV positive mothers in South Africa from receiving anti-retrovirals. It's estimated these policies led to the loss of more than 330,000 lives.
Still another example is so-called "climate change denialism", which "global warming/climate change" alarmists throw around cavalierly as an accusation against skeptics of "global warming/climate change". The alarmists who make such accusations attempt to conflate "climate change denialism" with actual Holocaust denial, which is highly offensive to Holocaust survivors and their families and could be considered a form of anti-Semitism, as those making accusations of "climate change denial" trivialize the Holocaust by deliberately daring to make such comparisons and then insensitively using them as accusations without considering their words or how they will affect others. When called out on their tactics, the "climate change" alarmists become defensive and self-righteous as they act offended while they dismissively accuse skeptics of "hysterical outrage" (a case of projection on the alarmists' part, as it is they themselves engaging in hysterical outrage over their offensive tactics being taken apart and called out) and, as liberals generally tend to do, refuse to take responsibility for their ill-considered comments.
The five most common characteristics of denialism
In order to assert their position and back up their claims, there are five commonly used characteristics often seen in the arguments or reasons given by those who are engaged in denialism.