J P A - Junior Philatelists of America.
Jet press - Another term for offset printing of stamps.
Joint issue - When a similar stamp design is issued by more than one country on the same day.
Joint line - A line formed by ink that accumulates between two printing plates.
Journal stamps - Stamps used specifically for prepaying postage on journals, newspapers, magazines, etc.
Jubilee - Stamp issues that feature a special (usually 25- year increment) anniversaries.
Judenpost - Ghetto stamps issued for the use of Jews interned in concentration camps.
Jumbo stamps - See Boardwalk Margins.
Jury - Judges at a stamp show.
Keytype - A basic stamp design used for the issues of two or more postal authorities which include captions of respective countries, denominations, etc.
Killer - Any obliterating postmark that’s used to cancel a stamp.
Killer bars - Horizontal lines used for stamp cancellations.
Kiloware - Collections (often sold by the kilo) made up of a variety of postally used stamps that are mounted on envelope corner paper.