noun1.a chemical substance that neutralizes alkalis, dissolves some metals, and turns litmus red; typically, a corrosive or sour-tasting liquid of this kind."rainwater is a very weak acid"2.CHEMISTRYa molecule or other entity that can donate a proton or accept an electron pair in reactions.adjective1.containing acid or having the properties of an acid; in particular, having a pH of less than 7."poor, acid soils"2.sharp-tasting or sour that from google .