1) Hard to get hold of 2) Hard to pin down 3) Indescribable 4) Intangible 5) Mysterious 6) Not definable 7) Not easily put into words 8) Obscure 9) Subtle 10) Undefinable 11) Undefined 12) Vague 1) Indescribable 2) Ineffable 3) Inexpressible 4) Insubstantial 5) Nameless 6) Undefinable 7) Undefined 8) Unexpressible 9) Unspeakable 10) Untellable 11) Vague 
• (a.) Incapable of being defined or described; inexplicable.
In`de·fin'a·ble adjective Incapable of being defined or described; inexplicable. Bp. Reynolds. 
[adj] - defying expression or description 2. [adj] - not capable of being precisely or readily described
indescribable adjective defying expression or description; `indefinable yearnings`; `indescribable beauty`; `ineffable ecstasy`; `inexpressible anguish`; `unspeakable happiness`; `unutterable contempt`; `a thing of untellable splendor`
not definable; not readily identified, described, analyzed, or determined.