This refers to the postage value given on a stamp such as 20p or £1.12.
FDC (First Day Cover)
This is a cover featuring new postage stamps, postmarked on the first day that those new stamps are on sale.
FDI (First Day of Issue) Postmark
The First Day of Issue postmark appeared for the very first time in slogan form in 1963, followed by a circular postmark for the 1964 Shakespeare stamps. Strictly speaking the term “FDI postmark” should only be used to describe postmarks sponsored by Royal Mail but the term is often more widely used to cover any cover with a pictorial postmark dated on the first day of a new set of stamps. Royal Mail reserve the right to the wording “First Day of Issue” so you will never find these words in a sponsored British postmark.
Since the earliest flights special covers have been carried on board to commemorate them. These are widely collected in their own right but on a few occasions they coincide with a new stamp coming out and so can be a ‘double’ first.
Frank or Franking
Another way of describing the postmark.