Facsimile - A likeness or imitation of a genuine stamp, marked in some way to denote its status so that it does not deceive a collector or defraud a postal administration. Catalog illustrations may be considered facsimiles.

Fake - A stamp, cover, or cancel that has been altered to appeal to a collector. In a broad sense, fakes include repairs, reperforations and regummed stamps, as well as painted-in cancels, bogus cancels, or counterfeit markings (this term is not synonymous with Forgery).

Fancy - (handstamp) - Postmark with a decorative design.

Faults - Factors that can decrease the value of a stamp. Factors include thin spots, creases, short or torn perforations, missing pieces, tears, or stains.

Feldpost - (German) Field Post Office.

Field Post Cards - A type of early postcard that was made available to soldiers operating in the field during wartime. They were first introduced in Prussia during the Franco-Prussian War in 1870 to provide a convenient way for soldiers to keep in touch with family. These cards were originally government issued, holding a printed stamp but no pictures as they were designed strictly for brief correspondence. In latter years, especially by World War 1, many private postcard publishers began producing field-post cards with imagery on them. This is best illustrated by the Feldpostkartes produced in Germany.

Field Post Office - (Military) - Abbreviation FPO, a place to which mail intended for military units in the field is sent to be sorted and forwarded.

First day cover - Abbreviation FDC, a newly-issued stamp affixed to an envelope and postmarked on the first day of sale.

First flight cover - A cover which has been carried on a first flight between two points and can be identified as such by a postmark or special cachet.

Fiscal - A revenue or tax stamp - not a postage stamp.

Flaw - Visible change in the printed design of a postage stamp due to damage to the printing surface of the plate or cylinder; 'constant flaws' are seen, either unchanged or very slowly progressing, over a large number of stamps printed by the same impression.

Flight cover - An envelope that has actually been flown.

Folded letter - Before the use of envelopes, letters that were written on a piece of notepaper that was folded after writing and sealed, generally by a wax seal.

Forgery - A fraudulent reproduction of a postage stamp or cover intended to deceive postal authorities and/or collectors.

Forwarding Agents Cachet - A strike (or endorsement) applied to a letter to indicate that it has been handled in transit by some means other than the Post Office.

Foxing - Refers to the brown spot-like stains sometimes found on paper. They can be caused by the oxidation of iron particles (ferric oxide) in the paper’s chemical residue left over from manufacturing. The most common cause of foxing is the action of enzymes from mold that has attached itself to accommodating impurities, and been encouraged to grow by a damp and warm environment. It is now believed that some of these brown stains are also a form of cellulose oxidation damage caused by repeated drying and wetting in environments with high fluctuations in humidity.

Franca - (Italian) Postage paid.

Franking - A mark on a cover showing that postage has been paid or that the envelope is being carried free of postage. Modern forms of franking are postage stamps and metered stamps.

Franco - (Italian) Postage paid.

Free mail - Mail sent without a fee due to various circumstances, like war, disaster, official mail, privileges granted and more.