Used to stop the rope from pulling through a cleat, hole or pulley.

Any terminal knot for preventing unreeving

A knot tied in the end of a rope or sling to keep something else from sliding off it

Or just "stopper"; a knot in the tail of a rope that prevents it from slipping through and keeps it out of the way. Used after tying the rope to your harness with a figure of eight, or on the your ropes before you abseil so you can't slip off the ends. Good stopper knots are the overhand and double overhand knots.