Illustrated Glossary of Organic Chemistry

Carbonate: (1) The anion formed by double deprotonation of carbonic acid, or a salt containing this anion.

Carbonic acid

Sodium carbonate
Deprotonation of carbonic acid (H2CO3) with two equivalents of base forms carbonate ion (CO32-). When NaOH is the base, the product is sodium carbonate (Na2CO3), a salt of the carbonate anion.

(2) An ester of carbonic acid.

General carbonate ester structure
Dimethyl carbonate
Ethyl phenyl carbonate