Entry 2 senses · 2 variants Webster, 1913 Lactonic · Lac·ton·ic · IPA a. a. 01 a. Of, pertaining to, or derived from, a lactone. 1. Of, pertaining to, or derived from, a lactone.(Chem.) 02 a. Pertaining to, or designating, an acid obtained by the oxidation of milk sugar (lactose). 1. Pertaining to, or designating, an acid obtained by the oxidation of milk sugar (lactose).(Chem.)