Meaning a stamp which has been deliberately printed or issued without perforations.   Typically imperforate stamps were printed between 1840-1850 these stamps might have been cut out from a sheet.   Since this time fewer imperforate stamps have been issued.   Stamps are now separated in a number of ways
  1. perforation: cutting rows and columns of small holes
  2. rouletting: small horizontal and vertical cuts
  3. die cutting cut paper to shape using a metal die