No-they do not expire. No US stamps issued since 1865 have been been devalued. The only exception are the special delivery stamps which paid for service no. longer offered. Air mail stamps are good for any purpose. Some stamps can't be used for certain things.
the pure and concentrated essence of a substance. 2. the most perfect embodiment of something. 3. (in ancient and medieval philosophy) the fifth essence or element, ether, supposed to be the constituent matter of the heavenly bodies, the others being air, fire, earth, and water. Origin of quintessence.
The quintessence of something is its perfect, ideal example. Some people say that the quintessence of American cuisine is the hamburger. Vegetarians may beg to differ. Ancient Greek philosophers claimed there were five elements: earth, water, air, fire, and a fifth substance that made up objects in the heavens.