Airmails - As the name suggests, these stamps were used to distinguish mail sent by air from that sent by truck or train. Now that the vast majority of mail is sent (at least part way) by air, they’re no longer issued. Hence, since their demise in the 1970s, they have some value as collectibles.

Approvals - Selections of stamps dealers offer to collectors ‘for approval’.

Authentication Mark - A mark that indicates an expert has examined and validated the stamp.

Block - A group of unseparated stamps. These can be valuable since they will have complete watermark designs and other design features that a single stamp would have only part of.

Centering - The position of a design on the stamp’s face relative to the margins. Since the manufacturing process may be imperfect, this can affect the value of a stamp.

Cover - The envelope, packaging, etc to which a stamp has been affixed.

Definitive - A regular issue of a stamp, produced at a specific rate of postage.

Denomination - The face value of a stamp.

Entire - An intact cover (envelope, packaging, etc).

Essay - The proposed design of a stamp, not yet issued.

Expertizing - The process of examination by an expert, such as the American Philatelic Expertizing Service.

First Day Cover - (FDC) An envelope or packaging to which the stamp is affixed and that bears a cancellation mark from the first day of issue.

Frame - The margins outside the area of the design of a stamp.

Freak - A stamp that has been incorrectly manufactured, whether from inking errors, perforation mistakes or other flaws.

Gutter - The selvage between the panes of a sheet of stamps. (See Selvage)

Hinge - Small, gummed rectangles of paper used to adhere stamps into an album by a narrow strip.

Imperforate - Stamps without perforations, intended to be separated using scissors.

Invert - Stamps printed with multiple colors in which one of the colored designs has been printed upside down. These can be valuable collectibles, such as the famous 1918 U.S. 24-cent Jenny.

Mint - A stamp in pristine condition, usually never used for postage. It has no hinge marks, the original adhesive is intact, and has no tears or marks.

Mission Mix - Originally meant on-paper stamps collected by churches or other charitable organizations and sold in great bulk to dealers to raise money for the charity. Same as kiloware or poundage.

NH - Never Hinged. The term refers to a stamp that has never been mounted by use of a hinge, the small adhesive mounts often used to adhere stamps into an album.

On Paper - Stamps that have been used for postage, typically still adhered to the cover (envelope, packaging, etc.).

OG - Original Gum. The original adhesive on the back of a stamp. How much remains, or has been damaged, can influence the value of a collectible stamp.

One Penny Black - The world’s first postage stamp, issued by Great Britain in 1840. Prior to that time recipients of mail often paid the postage, rather than the sender.

Packet - A collection or lot of stamps, they are usually comprised of all different stamps and grouped by topic or country. Can offer excellent value for collectors.

Pane - A sheet of unseparated stamps.

Perfins - Stamps with perforated initials, often issued to prevent work place theft .

Philately - The collection and study of stamps and related paraphernalia.

Proofs - Impressions taken from an approved die or plate, in which the design and color are the same as the stamps issued to the public.

Re-entry - Early plates showed wear quickly and images were often sharpened by rocking the transfer roll over the original impression. The slightest varation in registration would cause a doubling of parts of the design. These double images are called re-entries.

Selvage - The plain margin area around a sheet of stamps.

Se-tenant - Joined together, referring to an unseparated pair, strip or block of stamps differing in design, denomination or overprint.

Tagging - The use of phosphorescent dyes in stamp making. Machines can read the dye by use of an ultraviolet light and detect the face value and other elements.

Tete-beche - A pair of stamps in which one stamp is upside down in relation to the other.

Variety - A deviation from the standard issue. Differing watermarks, incorrect inks and other factors can produce a variety.

Watermark - An embedded design, used for anti-counterfeiting and to add esthetic value.