Source : Free On-Line Dictionary of Computing

built-in
     
        (Or "primitive") A built-in function or operator is one
        provided by the lowest level of a language implementation.
        This usually means it is not possible (or efficient) to
        express it in the language itself.  Typical examples are the
        basic arithmetic and {Boolean} operators (in {C} syntax: +, -,
        *, /, %, !, &&, ||), bit manipulation operators (~, &, |, ^)
        and I/O primitives.  Other common functions may be provided in
        libraries but are not built-in if they are written in the
        language being implemented.
     
        (1995-02-14)