1) too good, rare, special etc to be able to be replaced if lost, broken etc (irreplaceable)

2) for the present only and to be changed or replaced later (provisional)

3) a mechanical device used for controlling the flow of air, liquid, gas etc in one direction only; a structure in the heart or in a blood-vessel allowing the blood to flow in one direction only; a closed, airless tube used to control electricity (now replaced with transistors) (valve)