[syn: indefinable, indescribable, ineffable, unspeakable, untellable, unutterable]
The Collaborative International Dictionary of English v.0.48:
Indefinable \In`de*fin"a*ble\, a.
   Incapable of being defined or described; inexplicable. --Bp.
   Reynolds.
   [1913 Webster]
WordNet (r) 3.0 (2006):
indefinable
    adj 1: not capable of being precisely or readily described; not
           easily put into words; "an indefinable feeling of
           terror"; "an abstract concept that seems indefinable"
           [syn: indefinable, undefinable]
    2: defying expression or description; "indefinable yearnings";
       "indescribable beauty"; "ineffable ecstasy"; "inexpressible
       anguish"; "unspeakable happiness"; "unutterable contempt"; "a
       thing of untellable splendor" [syn: indefinable,
       indescribable, ineffable, unspeakable, untellable,
       unutterable]
Moby Thesaurus II by Grady Ward, 1.0:
72 Moby Thesaurus words for "indefinable":
   aleatoric, aleatory, amorphous, blobby, blurred, blurry, broad,
   chance, chancy, chaotic, confused, disordered,
   enwrapped in mystery, foggy, fuzzy, general, hazy, hit-or-miss,
   ill-defined, imprecise, inaccurate, inchoate, incoherent,
   incommunicable, indecisive, indefinite, indescribable,
   indeterminable, indeterminate, indistinct, ineffable, inenarrable,
   inexact, inexplicable, inexpressible, inextricable, innominable,
   insoluble, insolvable, lax, loose, mysterious, mystic,
   noncommunicable, nonspecific, obscure, orderless, random,
   shadowed forth, shadowy, shapeless, stochastic, sweeping,
   unaccountable, unclear, undefinable, undefined, undestined,
   undetermined, unexplainable, uninterpretable, unmentionable,
   unnameable, unplain, unsolvable, unspeakable, unspecified,
   untellable, unutterable, unwhisperable, vague, veiled