"A hack" in my book is a solution that works but is ugly, should probably be done better, but for one of various reasons (no time, no motivation, alternatives are awfully verbose and/or resource intensive) isn't. I tend to view these as sometimes necessary and always evils, that should be rooted out unless the hack in question is the lesser of evils.

I draw a very clear line between hacks and JAPHish/golfish trickery though. I have no fear of using the latter when it really does shorten the code by a noticable amount. Since I am generous with comments, I feel entitled to such tomfoolery.

Interestingly enough, I always have trouble bending my mind anew around a long forgotten hack that I have to take a second look at, but never any when trying to grok my long burried Golfish constructs.