The Collaborative International Dictionary of English v.0.48:

online \on"line`\, on-line \on"-line`\adj.
   1. 1 (Computers) connected by a communications line to a
      computer. Opposite of off-line.

   Syn: online, on line(predicate).
        [WordNet 1.5]

   2. [prenominal] being in progress now; ongoing; as, on-line
      editorial projects. [Colloq.]

   Syn: in progress(predicate), ongoing, on line(predicate).
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

on-line
    adj 1: on a regular route of a railroad or bus or airline
           system; "on-line industries" [syn: on-line, online]
           [ant: off-line]
    2: connected to a computer network or accessible by computer;
       "an on-line database" [syn: on-line, online] [ant: off-
       line]
    3: being in progress now; "on-line editorial projects" [syn:
       on-line(a), online]

The Free On-line Dictionary of Computing (30 December 2018):

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)