Source : Free On-Line Dictionary of Computing

hlp
     
         A {Microsoft Windows} {filename
        extension} for {hypertext} {WinHelp} files.  These are in a
        {proprietary} format, and are compiled from {source files}
        written in a dialect of {RTF}.
     
        See also {gid}.
     
        {Usenet} newsgroup:
        {news:comp.os.ms-windows.programmer.winhelp}.
     
        (1997-01-30)