A skeptic is defined as someone who reserves judgement on an issue until enough evidence is found to support a claim beyond a reasonable doubt and also clearly defines what would make him disbelieve a claim. This is scientific thinking.

By contrast, a denialist has no such defined limits, either of belief or disbelief. The denialist starts from a position of dogma, asserting opposition to an idea by presenting a contrary idea as absolute truth and interpreting all evidence according to that unalterable dogma, rather than draw conclusions based only on the evidence. This is the opposite of scientific thinking, although denialists often use scientific terminology to make their positions seem legitimate to fool the ignorant.

Denialism vs geuine skepticism is found in debates over evolution vs. Creationism, global warming, religion, and politics. If there were no denialists, most of those debates would have either ended long ago, or would be a lot more cordial than they tend to be.