Quiddity

1. (n.) The essence, nature, or distinctive peculiarity, of a thing; that which answers the question, Quid est? or, What is it?

2. (n.) A trifling nicety; a cavil; a quibble.

Thesaurus Entries

axiom, cavil, center, core, dodge, elixir, essence, essential, flower, focus, fundamental, gist, gravamen, heart, hypostasis, inner essence, jesuitism, kernel, marrow, meat, nub, nucleus, nuts and bolts, pith, postulate, principle, quibble, quid, quillet, quintessence, quip, quirk, quodlibet, sap, shuffle, soul, spirit, stuff, substance, the nitty-gritty