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.
If there's a particular time of day when your ten kittens settle down to nap, you can call that their period of quiescence, or the time when they are all quiet and restful. The noun quiescence looks similar to the word quiet for a good reason: quiescence is a quiet spell or state. It's used often to mean dormancy, or the quieting of a symptom or a disease, such as the quiescence of a child's asthma symptoms during the winter months. The Latin root word is quies, which means rest or quiet..