Source : Free On-Line Dictionary of Computing

RapidCAD
     
         A specially packaged {Intel 486DX} and a dummy
        {floating point unit} (FPU) designed as pin-compatible
        replacements for an {Intel 80386} processor and {80387} FPU.
        Since the DX variant has a working on-chip FPU, a dummy FPU
        package is supplied to go in the {387} FPU socket.
     
        (1995-05-10)