The Collaborative International Dictionary of English v.0.48:

Acidity \A*cid"i*ty\, n. [L. acidites, fr. acidus: cf. F.
   acidit['e]. See Acid.]
   The quality of being sour; sourness; tartness; sharpness to
   the taste; as, the acidity of lemon juice.
   [1913 Webster]

WordNet (r) 3.0 (2006):

acidity
    n 1: the property of being acidic [syn: sourness, sour,
         acidity]
    2: the taste experience when something acidic is taken into the
       mouth [syn: acidity, acidulousness]
    3: pH values below 7 [ant: alkalinity]