also: awayside · back side
The half of the formation the play is going away from. Its job is almost never to open anything. It is also where every piece of misdirection in football starts.
The side away from the point of attack, defined by where the ball is going rather than by left and right. The same lineman is backside on one call and frontside on the next without moving.
Backside linemen are rarely trying to move anybody. Their job is to cut off pursuit, which mostly means getting a body between a chasing defender and the ball rather than winning a block in any conventional sense. That is a genuinely different standard from the frontside, and a lineman judged by frontside standards on a backside assignment will look like he is losing when he is doing his job.
awaysideweakside, boundary, frontsideBackside is where misdirection lives. A backside guard who pulls is the whole mechanism of a trap and of a counter, and a backside defender who chases the fake is the player those plays are designed to punish. So a backside assignment is not automatically a low-value one: on the plays built around it, the backside IS the play.