carbonic acid

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A weak, unstable acid, H₂CO₃, present in solutions of carbon dioxide in water.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a weak unstable acid, H2CO3, known only in solution, and as carbonate salts; it is present in carbonated drinks, and sparkling wine, but decomposes to form carbon dioxide and water

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a weak acid known only in solution; formed when carbon dioxide combines with water