Standing in a connection to something else. Relation is a fact about the pair, never about either alone.
Built on relate, from Latin referre (to carry back, to relate): re- (back) + ferre (to carry); to be related is to stand in a carrying-back to something else, a fact about the pair rather than about either member alone.
Standing in a connection to something else. The root's carrying-back only exists between two things — relation is never a fact about one thing by itself.