It's often a synonym of "Spirit of the promotion". Draw your own conclusions.
What Balthazar said (even before the edit) is usually the case when players are accused of Bonus Abuse what a lot of casinos mean is players who use the casinos bonus in order to gain an advantage over them.
The truth is all decent bonuses which reward players with something worth having are open to be taken advantage of to some extent, if they weren't then they wouldn't be any good to the player.
So when many casinos label people "bonus abusers" they mean "advantage players" i.e players who play solely with bonuses and with the aim of winning money rather than for gambling as entertainment.
Of course if all players were like this bonuses would soon be shut off completely (bonuses are far less generous than they used to be because of advantage players).
it gets worse for the casinos when there are rings of advantage players - this happens a lot with welcome bonuses. A group of players will take the welcome bonus and bet high with the bonus money with the intention of hitting a big win, they will then move to a high RTP slot in order to grind out the wagering requirement before withdrawing. Casinos have moved to counter this by limiting max bets with bonus money to a set amount (in our case €5, discretionary of course we don't always apply it) or in some cases a set % of your balance (can get confusing).
Not really abuse but not something casinos can afford to get hit by all the time.
Some casinos will label you a bonus abuser if you only ever bet when they offer you a bonus. Again it's not really abuse, you're just taking an offer that they've given to you and maximising your advantage, but that happens a lot.
True bonus abuse only really occurs when there are multiple accounts, rings with dodgy cards/ID or fraudulent ID.
More often than not though it's something a casino manager will use to make up for the fact that he got his maths completely wrong and was battered due to a bad promotion.