The Free On-line Dictionary of Computing (30 December 2018):

POP-1

   Package for Online Programming.  Edinburgh, 1966.  First of
   the POP family of languages.  Used reverse Polish notation.
   Implemented as a threaded interpreter.  EPU-R-17, U Edinburgh
   (Jul 1966).  "POP-1: An Online Language", R. Popplestone, Mach
   Intell 2, E. Dale et al eds, Oliver and Boyd, Edinburgh 1968.