<p><strong>Noun:</strong></p><ul><li>The quality or state of being acid.</li><li>Sourness; tartness.</li><li>Excessive acid quality, as of the gastric juice.</li></ul><p><strong>Adjective:</strong></p><ul><li>Chemistry. Belonging or pertaining to acids or the anhydrides of acids.</li><li>Having only a part of the hydrogen of an acid replaced by a metal or its equivalent: an acid phosphate.</li><li>Having a pH value of less than 7.</li><li>Sharp or biting to the taste; tasting like vinegar; sour: acid fruits.</li><li>Sharp, biting, or ill-natured in mood, manner, etc.: an acid remark; an acid wit.</li><li>Geology. Containing much silica.</li><li>Metallurgy. Noting, pertaining to, or made by a process in which the lining of the furnace, or the slag that is present, functions as an acid in high-temperature reactions in taking electrons from oxide ions: usually a siliceous material, as sand or ganister.</li></ul>