When is a donk bet a donk bet?

As I understand it, a donk bet is when someone bets into the preflop raiser. It seems to me that this is not always wrong. Is the issue that if you feel you have true strength you should check-raise on the flop? Other than folding, this leaves checking and calling as the only option, which seems weak to me.

For example, let's say you call in early position with AJo, maybe get a few limpers, and a late position raise. If an ace hits the flop, your bet might force out weaker drawing hands and perhaps causing a fold from the late position raiser.

If a jack also fell on the flop I can see a check raise on the flop as the correct action.

So my question is basically when is a donk bet really a donk bet?

Comments?