The Collaborative International Dictionary of English v.0.48:

Positivity \Pos`i*tiv"i*ty\, n.
   Positiveness. --J. Morley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

positivity
    n 1: the character of the positive electric pole [syn:
         positivity, positiveness] [ant: negativeness,
         negativity]
    2: a quality or state characterized by certainty or acceptance
       or affirmation and dogmatic assertiveness [syn: positivity,
       positiveness, positivism] [ant: negativeness,
       negativism, negativity]
    3: the quality of being encouraging or promising of a successful
       outcome [syn: favorableness, favourableness,
       advantageousness, positivity, positiveness,
       profitableness] [ant: unfavorableness,
       unfavourableness]
    4: an amount greater than zero [syn: positivity,
       positiveness] [ant: negativeness, negativity]
    5: the quality of being undeniable and not worth arguing about
       [syn: incontrovertibility, incontrovertibleness,
       positivity, positiveness]