From The Free On-line Dictionary of Computing: (http://www.foldoc.org/, Editor Denis Howe)
The intermediate code produced by the Pascal-P compiler. Assembly language for a hypothetical stack machine, the P-machine, said to have been an imitation of the instruction set for the Burroughs Large System. The term was first used in Algorithms + Data Structures = Programs, N. Wirth, P-H 1976. Byte articles on writing a Pascal Compiler in Northstar BASIC (ca Aug 1978) also used the term. Later used in Apple Pascal, and as the intermediate language in the UCSD P-system.