Definition of cachets:
(n) :
(figurative) A special characteristic or quality; prestige, especially via association.
(n) :
(philately) A commemorative stamped design or inscription on an envelope, other than a cancellation or pre-printed postage.
(n) :
(medicine) A capsule containing a pharmaceutical preparation.
(v) :
(transitive, philately) To mark (an envelope) with a commemorative stamped design or inscription.
(n) :
A sealed envelope containing an item whose price is being negotiated.
(n) :
A hidden location from which one can observe birds while remaining unseen.