A band crush is when a person obsesses over a band/musician that they like. This could include constantly or repeatedly listening to their music within a short amount of time, to obessivly researching them (including personal lives.) In some extreme cases of band-crushing, the crusher will protect the crushee from any form of criticism whether from friends or others.
"I've read all about (insert band/musician here)! I even know their pet's names!"
"Dude, you've got a total band-crush going on"
"Hey, have you heard (insert band here)'s new single? it's a real let down"
"YOU TAKE THAT BACK!"
"Woahh man, don't have a band crush!"