What does it mean to be in denial?

In my opinion, to be in denial means acknowledging the truth of something in the presence of evidence for that something. I will use the example of smoking. There is ample evidence pointing to the fact that smoking causes lung cancer. Someone in denial would continue to smoke and think that lung cancer would never happen to them. Someone who simply disbelieved something would do so because the evidence is either invalid or questionable. I think someone who is in denial doesn't believe something for a different reason. Perhaps that reason is desire, a desire for that thing to be untrue.