Definition of "undefinable"

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.

1863, Sheridan Le Fanu, The House by the Churchyard

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.

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