Killer Postmark - any postmark that makes re-use of a stamp impossible
Kiss prints - marks that occur on sheet-fed presses when the paper accidentally contacts the inked plate before or after printing the stamps. As a rule, only a few marginal stamps will show the 'kiss'.
Knife - the cutting edge or 'cookie cutter', as philatelists refer to it, of the machine which cuts the envelope blank. Also the die-cut paper from which envelopes are folded.