quiddity

QUID'DITY, n. [L. quid, what.]

1. A barbarous term used in school philosophy for essence, that unknown and undefinable something which constitutes its peculiar nature, or answers the question, quidest? The essence of a thing constitutes it tale quid, such a thing as it is, and not another.

2. A trifling nicety; a cavil; a captious question.