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.
noun perÂ·sisÂ·tence \pÉr-Ësis-tÉn(t)s, -Ëzis-\. Tweet. : the quality that allows someone to continue doing something or trying to do something even though it is difficult or opposed by other people. : the state of occurring or existing beyond the usual, expected, or normal time.ull Definition of PERSISTENCE. 1. : the action or fact of persisting. 2. : the quality or state of being persistent; especially: perseverance. See persistence defined for English-language learners. See persistence defined for kids. ADVERTISEMENT.