The Collaborative International Dictionary of English v.0.48:

Exist \Ex*ist"\, v. i. [imp. & p. p. Existed; p. pr. & vb. n.
   Existing.] [L. existere, exsistere, to step out or forth,
   emerge, appear, exist; ex out + sistere to cause to stand, to
   set, put, place, stand still, fr. stare to stand: cf. F.
   exister. See Stand.]
   1. To be as a fact and not as a mode; to have an actual or
      real being, whether material or spiritual.
      [1913 Webster]

            Who now, alas! no more is missed
            Than if he never did exist.           --Swift.
      [1913 Webster]

            To conceive the world . . . to have existed from
            eternity.                             --South.
      [1913 Webster]

   2. To be manifest in any manner; to continue to be; as, great
      evils existed in his reign.
      [1913 Webster]

   3. To live; to have life or the functions of vitality; as,
      men can not exist in water, nor fishes on land.

   Syn: See Be.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

existing \existing\ adj.
   1. having existence or being or actuality; as, much of the
      beluga caviar existing in the world is found in the Soviet
      Union and Iran. Opposite of nonexistent. [Narrower
      terms: active, alive; extant, surviving] Also See:
      extant.

   Syn: existent.
        [WordNet 1.5]

   2. Present. Opposite of absent.
      [WordNet 1.5]

   3. Presently existing; as, the existing system.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

existing
    adj 1: presently existing; "the existing system"
    2: having existence or being or actuality; "an attempt to refine
       the existent machinery to make it more efficient"; "much of
       the beluga caviar existing in the world is found in the
       Soviet Union and Iran" [syn: existent, existing] [ant:
       nonexistent]
    3: existing in something specified; "depletion of the oxygen
       existing in the bloodstream"

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "existing":
   actual, alive, around, as is, being, contemporaneous, contemporary,
   current, existent, extant, fresh, immanent, immediate, in being,
   in effect, in existence, in force, instant, latest, living, modern,
   new, on foot, present, present-age, present-day, present-time,
   prevalent, running, subsistent, subsisting, that be, that is,
   topical, under the sun, up-to-date, up-to-the-minute