A collection defined by membership alone. What matters is what is in it, not the order or the arrangement.
Old English settan (to set, place), from Proto-Germanic *satjan, a causative of sit, merged in later English with Old French sette (a following, a group), from Latin secta (from sequi, to follow) — one spelling grown from two separate words.
A collection known only by what belongs to it — order and arrangement do not matter here, only membership does.