Opening range
The set of starting hands worth raising first in from a given position, with nobody else having entered the pot yet.
Opening ranges widen as fewer players are left to act behind you, since every player still to come is another chance for someone to wake up with a strong hand and punish a marginal open. Under the gun, first to act at a full table, opens only the top of the deck; the button, last to act with guaranteed position for the rest of the hand, opens almost anything with two live cards. The same two cards can be a clear fold from one seat and a routine open from another.