The Collaborative International Dictionary of English v.0.48:

Carbonic \Car*bon"ic\, a. [Cf. F. carbonique. See Carbon.]
   (Chem.)
   Of, pertaining to, or obtained from, carbon; as, carbonic
   oxide.
   [1913 Webster]

   Carbonic acid (Chem.), an acid HO.CO.OH, not existing
      separately, which, combined with positive or basic atoms
      or radicals, forms carbonates. In common language the term
      is very generally applied to a compound of carbon and
      oxygen, CO2, more correctly called carbon dioxide. It
      is a colorless, heavy, irrespirable gas, extinguishing
      flame, and when breathed destroys life. It can be reduced
      to a liquid and solid form by intense pressure. It is
      produced in the fermentation of liquors, and by the
      combustion and decomposition of organic substances, or
      other substances containing carbon. It is formed in the
      explosion of fire damp in mines, and is hence called
      after damp; it is also know as choke damp, and
      mephitic air. Water will absorb its own volume of it,
      and more than this under pressure, and in this state
      becomes the common soda water of the shops, and the
      carbonated water of natural springs. Combined with lime it
      constitutes limestone, or common marble and chalk. Plants
      imbibe it for their nutrition and growth, the carbon being
      retained and the oxygen given out.

   Carbonic oxide (Chem.), a colorless gas, CO, of a light
      odor, called more correctly carbon monoxide. It is
      almost the only definitely known compound in which carbon
      seems to be divalent. It is a product of the incomplete
      combustion of carbon, and is an abundant constituent of
      water gas. It is fatal to animal life, extinguishes
      combustion, and burns with a pale blue flame, forming
      carbon dioxide.
      [1913 Webster]

WordNet (r) 3.0 (2006):

carbonic
    adj 1: relating to or consisting of or yielding carbon [syn:
           carbonaceous, carbonous, carbonic, carboniferous]