Strictly, a mosher is any person who "moshes"; a type of pseudo-dance characterized by jumping around wildy, pushing, shoving, and kicking within the bounds of a "mosh pit."
More specifically, a mosher is a person who listens to any genre of music ranging from punk to metal. They are often associated with antisocial behavior as well as other behavioral disorders.
In addition, they characterize themselves with specific styles of dress which they claim to be unique, different, original, our outside of the "norm." This is regardless of the fact that their dress styles are sterotypical and faddish, demonstrate a strict conformity to the "mosher" subculture, and are completely within the cultural norm of this group.
Mosher 1 - Lets put black makup on our eyes to be rebellious and show that we don't conform to society!
Mosher 2 - Great idea! Then lets go to a mosh pit and playfight with 100 other people who are wearing the EXACT SAME black makeup! Because we're non-conformists! Hardcore!