Generic jam band music; typically garden variety rock bands that cater to the hippie crowd, generally adhering to a long format, jam session style of music. Merging modern rock, jazz, funk, afrobeat, and many other genres, gen jam is a diluted style of music and many bands often sound similar - it may be difficult for a "square" listener to differentiate between bands. Examples would include: Phish, moe., Umphrey's McGee, O.A.R., Dave Matthews Band, The String Cheese Incident, and Widespread Panic.
Stoner 1: "Hey man, are you going to Moe tonight at the Orpheum?"
Stoner 2: "Nah, man. Gen jam is hardly worth fifty bucks at the door. I'm headed to Chicago to catch The New Deal tonight anyway."
Stoner 1: "Chill."