The Collaborative International Dictionary of English v.0.48:

Quiddity \Quid"di*ty\, n.; pl. Quiddities. [LL. quidditas, fr.
   L. quid what, neut. of quis who, akin to E. who: cf. F.
   quiddit['e].]
   1. The essence, nature, or distinctive peculiarity, of a
      thing; that which answers the question, Quid est? or, What
      is it? " The degree of nullity and quiddity." --Bacon.
      [1913 Webster]

            The quiddity or characteristic difference of poetry
            as distinguished from prose.          --De Quincey.
      [1913 Webster]

   2. A trifling nicety; a cavil; a quibble.
      [1913 Webster]

            We laugh at the quiddities of those writers now.
                                                  --Coleridge.
      [1913 Webster]

WordNet (r) 3.0 (2006):

quiddity
    n 1: an evasion of the point of an argument by raising
         irrelevant distinctions or objections [syn: quibble,
         quiddity, cavil]
    2: the essence that makes something the kind of thing it is and
       makes it different from any other [syn: quiddity,
       haecceity]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "quiddity":
   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