Peter Boghossian characterizes faith as "pretending to know something you don't know".
Many religious people don't like this. They say they aren't pretending, they "feel it in their hearts".
So I suggest as an alternate, "faith is feeling in your heart that you know something you don't know".
You could defend PB's definition of faith. Perhaps after believers deconvert, they would see they had been pretending in some sense. Not consciously pretending, but they suppressed or rationalized their pretending.
This is perhaps true for the majority of believers. Others are just plain delusional - truly convinced and not pretending at all.
I think this is a better way to describe it to believers. As an atheist, you can clearly see that they are claiming to know something completely unknowable, and therefore it would be correct to say they are pretending to know. But believers legitimately believe they know these things, and most think they have legitimate reasons. I did when I was a believer. It seems to me most people of faith don't really know what faith is though. What I call faith is not what others call faith. One believer I talked to said faith is the expectation for something to happen if that specific thing hasn't happened before. His example was "if you are standing on train tracks and a train is coming at you, you would jump out of the way because you have faith that the train will hit you even though you've never been hit by a train before." He really could not see how that was a gross misreresentation of what faith is. If we are going to convince people that faith is unreliable, we first need to figure out how to show them that faith and reasoning are not the same thing. For me, it is so obvious a concept that I don't know how to simplify it enough to explain it to someone who doesn't realize this. It's like trying to explain that the sky is blue to someone who thinks blue is green.
I emailed PB and he replied "or 'you can't know'"
i.e. "faith is feeling you know something you can't know".
I like that except I'm not sure how we know someone can't know that God exists.