Source : Free On-Line Dictionary of Computing

on-line
     
         1. Ready for use.  E.g. "The graph plotter's fixed
        and on-line again".
     
        2. {Interactive} as opposed to {batch}.  Accessible via a
        computer (or {terminal}), rather than on paper or other
        medium.
     
        3. Of a user, actively using a computer system, especially the
        {Internet}.  E.g "I haven't been on-line for three days."
     
        "On-line" should be hyphenated because it is compounded from
        two words but the hyphen is often omitted in names of
        organisations or services.
     
        (1998-12-22)