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.
   • 2006, Sean Lennon, Paul Spickard, Kip Fulbeck, Part Asian, 100 Per Cent Hapa:

     What's interesting is ambiguity. What's interesting is the haziness, the blurrings, the undefinables, the space and tension between people, the area between the margins that pushes us to stop, to question.