cachet

Short definition from Wiktionary

Noun

1. 1
   A seal, as of a letter.
2. 2
   A special characteristic or quality; prestige, especially via association.
3. 3
   A commemorative stamped design or inscription on an envelope, other than a cancellation or pre-printed postage.
4. 4
   A sealed envelope containing an item whose price is being negotiated.
5. 5
   A capsule containing a pharmaceutical preparation.
6. 6
   A hidden location from which one can observe birds while remaining unseen.

Verb

1. 1
   To mark (an envelope) with a commemorative stamped design or inscription.