One who zealously classifies music as being of a certain genre.
A genre nazi will argue against anyone who claims the music in question is a different genre than what the grammar nazi has said it to be.
The word "grammar nazi" stems from the
slang term for someone who aggresively corrects
people on their spelling, vocabulary and grammar - a "Grammar Nazi".
Typical genre nazi argument.
"Deadmau5 is a progressive house/trance artist, not trance/progressive house artist, you
idiot!"
"Metallica are a thrash metal/speed metal band, not a speed metal/thrash metal band, you
idiot!"
"Rihanna is a R&B/pop music singer, not a pop music/R&B singer, you
idiot!"