WordWeb Online

1. Not capable of being precisely or readily described; not easily put into words
   "an abstract concept that seems indefinable"; "an indefinable feeling of terror";
   - undefinable
    
2. Defying expression or description
   "indefinable yearnings";
   - indescribable, ineffable, unspeakable, untellable, unutterable, undescribable

See also: inexpressible, undefined, unexpressible, vague

Encyclopedia: Indefinable