Source : WordNet®

copybook
     n : a book containing models of good penmanship; used in
         teaching penmanship

Source : Free On-Line Dictionary of Computing

copybook
     
         (Or "copy member", "copy module") A
        common piece of {source code} designed to be copied into many
        source programs, used mainly in {IBM} {DOS} {mainframe}
        programming.
     
        In {mainframe} {DOS} (DOS/VS, DOS/{VSE}, etc.), the copybook
        was stored as a "book" in a {source} library.  A library was
        comprised of "books", prefixed with a letter designating the
        language, e.g., A.name for Assembler, C.name for Cobol, etc.,
        because {DOS} didn't support multiple libraries, private
        libraries, or anything.  This term is commonly used by {COBOL}
        programmers but is supported by most {mainframe} languages.
        The {IBM} {OS} series did not use the term "copybook", instead
        it referred to such files as "libraries" implemented as
        "partitioned data sets" or {PDS}.
     
        Copybooks are functionally equivalent to {C} and {C++}
        {include} files.
     
        (1997-07-31)