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 the play's direction rather than by left and right, so the same lineman can be frontside on one call and backside on the next.
Blocking rules use frontside and backside so the same rule can apply when a play is called in either direction.
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.