Source : Free On-Line Dictionary of Computing

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.