1. Someone who opposes or declares that something is not true: Deniers are individuals who take pleasure in disputes.

One political person is a climate change denier who evidently doesn't understand that this has scientifically been proven as taking place.
2. Anyone who refuses to let someone have or to do something: Since her mom was a denier, Susan wasn't allowed to play outside until her homework was done!
3. A person who refuses to gratify his or her own needs or desires: Grace turned out to be her own denier when she decided not to go on the cruise around the Mediterranean Sea because she didn't want to get into debt because of it.