a. A person with a truly amazing voice.
b. A role model for young girls that suggests that being a dependent, whiny, self-obsessed, arrogant bitch who indulges in self-victimization (it helps to sell records, god damn it) makes a woman super-duper-hawt. In other words, a feminist's nightmare come true.
Most people know at least two or three Amy Lees.
If your Amy Lee next door is not fake-crying, reading Twilight, taking pictures of herself fake-crying and reading Twilight, photoshopping the mentioned pictures in a ridiculously shitty way or posting them on a social network, then she probably went to that club in order to be bored and look down on everyone who is dumb enough to have fun.
Jhonen Vasquez, creater of Invader Zim and Johnny The Homicidal Maniac, is also one of the first describers of the stereotypical Amy Lee (see "Anne Gwish").
If you think I am talking about goth girls in general, read the definition again, but with your brain on.