The Collaborative International Dictionary of English v.0.48:

Existential \Ex`is*ten"tial\, a.
   1. Having existence. [Archaic] --Bp. Barlow.
      [1913 Webster]

   2. of or pertaining to, or having the character of,
      existentialism.
      [PJC]

   3. (Logic) specifying actual existence, rather than only
      possibility; as, the existential operator.
      [PJC] --Ex`is*ten"tial*ly, adv. [Archaic]
      [1913 Webster]

            Existentially as well as essentially intelligent.
                                                  --Colerige.
      [1913 Webster]

WordNet (r) 3.0 (2006):

existential
    adj 1: derived from experience or the experience of existence;
           "the rich experiential content of the teachings of the
           older philosophers"- Benjamin Farrington; "formal
           logicians are not concerned with existential matters"-
           John Dewey [syn: experiential, existential]
    2: of or as conceived by existentialism; "an existential moment
       of choice"
    3: relating to or dealing with existence (especially with human
       existence)

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "existential":
   Cyrenaic, Eleatic, Epicurean, Megarian, Stoic, animist, animistic,
   atomistic, cosmotheistic, eclectic, empirical, eudaemonistic,
   hedonic, hedonist, hedonistic, humanist, humanistic, idealistic,
   instrumentalist, materialistic, mechanistic, metaphysical,
   monistic, naturalistic, nominalist, panlogistical, pantheistic,
   positivist, positivistic, pragmatic, pragmatist, rationalistic,
   realist, realistic, scholastic, sensationalistic, syncretistic,
   theistic, transcendentalist, transcendentalistic, utilitarian,
   vitalistic, voluntarist, voluntaristic