Source : Webster's Revised Unabridged Dictionary (1913)

Haecceity \H[ae]c*ce`i*ty\, [L. h[ae]cce this.] (Logic)
   Literally, this-ness. A scholastic term to express
   individuality or singleness; as, this book.

Source : WordNet®

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