One of the bounding faces of a thing, and one of the parties in a matter. Both are positions relative to a middle.
Old English side (the flank of a body or an object), from a Proto-Germanic root of uncertain further origin; one of the bounding faces of a thing, or one of the parties in a dispute.
A position that only exists because at least one other position stands opposite it — naming a side, of an object or of an argument, is already naming the middle it sits against.