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)