The bible defines faith as "the assured expectation of things hoped for, the evident demonstration of realities, though not beheld."
In other words, it is the belief that what one wants to have as reality is reality, even if you can't see it. For example, JWs refer to Jesus' parousia beginning in 1914, when Jehovah made him the King of his invisible heavenly government. The WTS says this is true, and JWs believe it to be true, because that's what they have been told is The Truthâ„¢. All this without a single shred of evidence.
My medical dictionary defines delusions as "fixed false beliefs that persist despite evidence to the contrary".
Basically "faith" can exist in the complete lack of evidence, just like delusions do. The only difference is that when someone claims that their fixed false beliefs that persist despite evidence to the contrary are the result of religious faith, we have been conditioned to accept this as a valid rationale for holding irrational, non-evidence based beliefs. Anyone else gets referred to a psychiatrist.