What is a cliché?

Here is an example related to French philately 

"...uneven plate wear was always a problem, and the plates that these early French stamps were produced on were groups of 100 clichés (“cliché” is what each individual stamp die was called) held together in rows. When one cliché was damaged, it could be replaced by itself leaving the stamp cliché around it intact. This produced some of the most stunning varieties in philately, found on many early French stamps and on very few other classic stamps of the world. When the printers replaced a cliché, if the cliché was placed upside down, the result was two stamps that were head to tail next to each other (called “tête-bêche” in French). Not only were tête-bêche stamps unusual—the misplacement of the clichés was unusual in itself and was fixed as soon as it was discovered—but to be collected, a large block had to be saved from just the right part of the sheet or a pair had to be used on a cover."


You can read more about it HERE

Further to Stan's assistance,

Basically  "copies of stamp dies, arranged in order to form a plate of stamp images"

How these copies (cliches) were made, is a fascinating study, of all countries
that used them.

Polytyping the origination of Cliche (Postage Stamps in the making : Easton)



Cliché: end result of the process of applying a design into metal; individual identical units that are used to make up a
printing plate, plate, image, block, photo, line cut, position.


Resetting: a new arrangements of cliches in a plate that produces a tete-beche layout.

Rouletted in color: notched rules are put between the cliches forming the plate from which the stamps are printed, and
these rules being inked with the plate, the edges of which are colored.

Set : stamp production term of composing type or the arrangement of cliches so that sheet of stamps can be printed in that
arrangement.

Setting error: occurs when a cliche is inserted sideways or upside down in relation to the rest of the plate.

Substitute cliché: a single cliché inserted into a printing plate in place of a damaged one; can be identified if it fails to line
up exactly with those around it.