also: playside · play side · front side
The half of the formation the play is attacking. It is defined by where the ball ends up rather than by left and right, which is why nearly every blocking rule in football is written in terms of it.
The side of the formation the play is designed to attack, also called playside. It is defined by where the ball ends up rather than by left and right, so the same lineman is frontside on one call and backside on the next without moving.
That is the reason blocking rules are written this way rather than in left and right. A rule that names the frontside guard survives the play being run to either side, being run out of a flipped formation, and being run by a different personnel group. A rule that names the right guard has to be rewritten for every one of those.
playsidestrongside, field side, backsideFrontside and backside describe the PLAY. Strongside and weakside describe the FORMATION, and field and boundary describe where the ball sits on the hash. All three pairs split the field in two and they routinely disagree with each other: a play run to the weakside is going frontside to the weak side, which sounds like a contradiction and is not one. When a rule sounds contradictory, check which of the three pairs it is using.
Worth teaching explicitly at youth level, where linemen are often taught left and right instead. The cost of left and right shows up the first time the offense runs the same play to the other side and every rule has to be re-learned.