What defines a "damsel in distress"?

So at what point does a heroine stop being "plucky young hero defying the odds" and become "damsel in distress"? How do we avoid this trope? What defines a "damsel in distress"?

For example, I have a female character who is small, not particularly strong, but very brave and eager to do the right thing. She is capable enough, clever and good with a bow, but her skills are more about persuasion and compassion, but she doesn't give up when that fails. As such, she's always diving headfirst into danger to do the right thing.

Her male companion/eventual lover, who is a somewhat better fighter (vampire) although not as good as he thinks, is far less inclined to help people but is constantly plunging into danger with her - not because he cares, but to make sure she isn't killed. She does save him sometimes, but usually he's saving her, or they're getting through together by the skin of their teeth.

That's just, I think, a consequence of me putting a poor fighter and a mid fighter into situations that really require someone who is a BETTER fighter, and they only survive by working together. Because if they can easily defeat their foe, how is that fun, or interesting?

I'm beginning to think this may just be an intrinsically male v female thing - ie a plucky young lad and his big brave sidekick would not be accused of "damselling" when he gets in over his head. Is it just a consequence where any female, put in a situation where she is out of her depth and needs help, becomes a D.I.D.?