The Collaborative International Dictionary of English v.0.48:

off-line \off-line\ adj.
   1. (Computers) Not connected; -- of computers or computer
      peripherals normally connected or intended to be connected
      to other computers by a communications line; as, we can't
      print the document because the printer is off-line.
      [Narrower terms: disconnected, not ready, off;
      unconnected ] {on-line

   Syn: offline, off line(predicate).
        [WordNet 1.5]

   2. Hence: (fig.) Outside of or after a meeting or formal
      discussion; as, we can discuss the details off-line; -- a
      term used at meetings and conferences to suggest
      postponing detailed discussion of a topic so as not to
      occupy the time of a large group most of whom may not be
      interested.
      [PJC]

The Jargon File (version 4.4.7, 29 Dec 2003):

offline
 adv.

    Not now or not here. ?Let's take this discussion offline.? Specifically
    used on Usenet to suggest that a discussion be moved off a public
    newsgroup to email.