<jargon> 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.
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Nearby terms: One Time Programmable Read-Only Memory « one-way function « one-way hash function « on-line » On-Line Analytical Processing » Online Computer Library Center, Inc. » On-Line Computer system