ester [es´ter] a compound formed from an alcohol and an acid by removal of water. es·ter (es'tĕr), An organic compound containing the grouping, -X(O)-O-R (X = carbon, sulfur, phosphorus, etc.; R = radical of an alcohol), formed by the elimination of H2O between the -OH of an acid group and the -OH of an alcohol group; usually written as in ethyl ...