Dictionary defines *denial* in a few senses, the ones I'm interested in are:

> 
> * refusal to believe a doctrine, theory, or the like.
> * disbelief in the existence or reality of a thing.
> * refusal to recognize or acknowledge.
> 
> 
> 

In particular, I mean unreasonable disbelief in reality of a thing which is definitely, objectively real.

Now, I want to say:

> 
> Denial is a horrible [what?]
> 
> 
> 

trait of character? (not really, it's specific, not encompassing whole personality) vice? (same) belief? idea? (no, it's a property of a belief or idea...)

How would you classify it?