The Collaborative International Dictionary of English v.0.48:

Existence \Ex*ist"ence\, n. [Cf. F. existence.]
   1. The state of existing or being; actual possession of
      being; continuance in being; as, the existence of body and
      of soul in union; the separate existence of the soul;
      immortal existence.
      [1913 Webster]

            The main object of our existence.     --Lubbock.
      [1913 Webster]

   2. Continued or repeated manifestation; occurrence, as of
      events of any kind; as, the existence of a calamity or of
      a state of war.
      [1913 Webster]

            The existence therefore, of a phenomenon, is but
            another word for its being perceived, or for the
            inferred possibility of perceiving it. --J. S. Mill.
      [1913 Webster]

   3. That which exists; a being; a creature; an entity; as,
      living existences.
      [1913 Webster]

WordNet (r) 3.0 (2006):

existence
    n 1: the state or fact of existing; "a point of view gradually
         coming into being"; "laws in existence for centuries" [syn:
         being, beingness, existence] [ant: nonbeing,
         nonentity, nonexistence]
    2: everything that exists anywhere; "they study the evolution of
       the universe"; "the biggest tree in existence" [syn:
       universe, existence, creation, world, cosmos,
       macrocosm]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "existence":
   accessibility, actual presence, actuality, an existence,
   animal spirits, animate existence, animation, availability, being,
   being alive, being here, being there, birth, body, continuance,
   continuation, creature, critter, duration, endurance, ens,
   entelechy, entity, essence, existent, fact, having life, hereness,
   immanence, immediacy, immortality, individual, individuality,
   indwellingness, inherence, life, lifetime, liveliness, living,
   long life, longevity, monad, object, occurrence, organism,
   permanence, persistence, person, persona, personality,
   physical presence, presence, quiddity, reality, something, soul,
   spiritual presence, spriteliness, thereness, thing, ubiety, unit,
   viability, vitality, vivacity, whereness



The Devil's Dictionary (1881-1906):

EXISTENCE, n.

    A transient, horrible, fantastic dream,
    Wherein is nothing yet all things do seem:
    From which we're wakened by a friendly nudge
    Of our bedfellow Death, and cry:  "O fudge!"