A surprise or planned fight, when either (a)one individual is attacked/beaten by a group of 2 or more individuals or (b) when a notiably larger group attacks/fights a smaller group of individuals. These fights can occur with or without any kind of valid reason to jump someone.
A onesided fight.
"Man lets jump this kid coming this way on his bike"
"Me and my cousin are going to jump this guy that stole our car"