Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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