The Collaborative International Dictionary of English v.0.48:

Acidic \A*cid"ic\, a. (Min.)
   1. Containing a high percentage of silica; -- opposed to
      basic.
      [1913 Webster]

   2. of or relating to acid; having the character of an acid,
      as an acidic solution.
      [PJC]

WordNet (r) 3.0 (2006):

acidic
    adj 1: being or containing an acid; of a solution having an
           excess of hydrogen atoms (having a pH of less than 7)
           [ant: alkalic, alkaline, amphiprotic, amphoteric]
    2: being sour to the taste [syn: acidic, acid, acidulent,
       acidulous]