undefinable

un- +‎ definable

undefinable (comparative more undefinable, superlative most undefinable)

1. Not definable.
   • 1863, Sheridan Le Fanu, The House by the Churchyard

     There was some little undefinable coolness between old General Chattesworth and Devereux. He admired the young fellow, and he liked good blood in his corps, but somehow he was glad when he thought he was likely to go.

• indefinable (more common)

• undefinability

undefinable (plural undefinables)

1. Anything that cannot be defined.