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]

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!"