The Collaborative International Dictionary of English v.0.48:

Haecceity \H[ae]c*ce"i*ty\ (h[e^]k*s[=e]"[i^]*t[y^]), [L.
   h[ae]cce this.] (Logic)
   Literally, this-ness. A scholastic term to express
   individuality or singleness; as, this book.
   [1913 Webster] Haemato
   Haema

WordNet (r) 3.0 (2006):

haecceity
    n 1: the essence that makes something the kind of thing it is
         and makes it different from any other [syn: quiddity,
         haecceity]