A hand-stamp on an envelope or card to denote the special circumstances in which it has been posted, eg. on an important expedition , on an air flight, at an exposition, the first day use of a stamp or a particular anniversary or event. It may be official (applied by the Post Office) or private (independent of Post Office authority).
Cachets are also small marks stamped on the backs of stamps by experts, dealers and collectors as marks of guarantee, authenticity or identification. Generally made in the form of initials.
