The term "band geek" refers to a high school and college subculture found primarily in the United States. It involves the students who are heavily involved in marching bands and/or concert band, for example.
Band geek culture develops around the students' schedule of rehearsals, performances, and competitions. Band geeks can also participate in other groups like jazz band, winter drumline, and winterguard. Additionally, due to the amount of time spent together, members of color guard have also been subsumed into the "band geek" group, although more specific terms, such as "flag nerd" "flagget" or "saber geek" also exist. The term Band Nerd, which was at one time correct, can be used as a thing to enrage and/or egg on said "band geeks."
"Band geek" can also be used as a blanket term for any student in middle or high school who plays an instrument and are in a band class, including students in the orchestra, or even those who play an instrument outside of school (assuming it's an instrument typically found in a concert band or orchestra). A comparable term applied to orchestra members is "orch dork", and band geeks often use the name as a derogatory term.