An insecure child who tries very hard to be something besides being themselves.
Nowadays, most kids are posing as punks/goths/emos/skaters by dressing like them, acting like them, and sharing the same interests as them. Since so many kids are doing this, its very difficult to tell the difference between a real goth and a poser goth.
Here's a more specific definition:
Posers can be quite vicious. They pick on many people, most of who haven't done anything to them.
VICTIM #1: Real punks/goths/emos/skaters. Posers usually call them posers because they want a self-esteem boost. They want to be more goth than the goths, or more punk than the punks are. But lemme tell you posers out there, if you posing as a punk( Im going to use punk as an example because there are more punk posers than other posers) and the only reason you think you are is because you buy all of your apparel at Hot Topic and think Good Charlotte is cool only because you think Benji Madden looks hot in liberty spikes, you have no right to call someone who is a real punk, someone who is really anti-establishment and is all about the music, a poser.
VICTIM #2: Anyone else who is not trying to be punk/goth/emo/skater and is interested in the same things as they are. For example, if a poser who thinks he/she is a goth because they wear no other color but black sees someone wearing black capris, they call them a poser because apparently to them, goths are the only people that are ever allowed to wear black. So what if they like black too! Goths aren't the only ones who are allowed to wear black.
So basically, a poser is someone who claims they are punk/emo/goth/skater and go around calling other people with the same interests as them posers because they claim they're "stealing their interests" and also to make themselves seem more punk/emo/goth/skater.
I hope my furious rant plainly explained to you what a true poser is.
One poser called me a poser just because I like checkers and the color red. That is a load of bullshit.