[1] : The Collaborative International Dictionary of English v.0.48

quiddity \quid"dity\, 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
see also:
quiddities 

[2] : WordNet (r) 2.0

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

[3] : 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

Quiddity >> Q Words
Quiddity, definition of term: Quiddity
quiddity_pag1.html