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]

Moby Thesaurus II by Grady Ward, 1.0:

108 Moby Thesaurus words for "acidity":
   acerbity, acescency, acid, acidness, acidulousness, acridity,
   acrimony, agent, alkali, alkalinity, alloisomer, animosity, anion,
   antacid, asperity, astringency, atom, base, bile, biochemical,
   bite, bitingness, bitter resentment, bitterness,
   bitterness of spirit, cation, causticity, causticness, chemical,
   chemical element, choler, chromoisomer, compound, copolymer,
   corrosiveness, cuttingness, dimer, dryness, edge, element,
   fierceness, gall, gnashing of teeth, greenness, grip,
   hard feelings, harshness, heartburning, heavy chemicals,
   high polymer, homopolymer, hydracid, hyperacidity, incisiveness,
   inorganic chemical, ion, isomer, keenness, macromolecule, metamer,
   molecule, monomer, mordacity, mordancy, neutralizer, nonacid,
   organic chemical, oxyacid, piercingness, piquancy, poignancy,
   point, polymer, pseudoisomer, pungency, radical, rancor, rankling,
   reagent, rigor, roughness, severity, sharpness, slow burn,
   soreness, sour, sourishness, sourness, spleen, stabbingness, sting,
   stridency, stringency, subacidity, sulfacid, tartishness, tartness,
   teeth, trenchancy, trimer, unripeness, unsweetness, vehemence,
   verjuice, vinegariness, vinegarishness, violence, virulence