<p><strong>Noun:</strong></p><ul><li>Chemistry. A compound usually having a sour taste and capable of neutralizing alkalis and reddening blue litmus paper, containing hydrogen that can be replaced by a metal or an electropositive group to form a salt, or containing an atom that can accept a pair of electrons from a base.</li><li>Acids are proton donors that yield hydronium ions in water solution, or electron-pair acceptors that combine with electron-pair donors or bases.</li><li>A substance with a sour taste.</li><li>Something, as a remark or piece of writing, that is sharp, sour, or ill-natured: His criticism was pure acid.</li><li>Slang. LSD.</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>