ac·id (?s??d)n.1. Chemistry a. Any of a class of substances whose aqueous solutions are characterized by a sour taste, the ability to turn blue litmus red, and the ability to react with bases and certain metals to form salts.b. A substance that yields hydrogen ions when dissolved in water.c. A substance that can act as a proton donor.d. A substance that can accept a pair of electrons to form a covalent bond.2. A substance having a sour taste.3. The quality of being sarcastic, bitter, or scornful: wrote with acid about her first marriage.4. Slang See LSD1.adj.1. Chemistry a. Of, relating to, or containing an acid.b. Having a high concentration of acid.c. Having the characteristics of an acid.2. a. Having a pH of less than 7.b. Having a relatively high concentration of hydrogen ions.3. Geology Containing a large proportion of silica: acid rocks.4. Having a sour taste. See Synonyms at sour.5. Biting, sarcastic, or scornful: an acid wit; an acid tone of voice.[From Latin acidus, sour, from ac?re, to be sour; see ak- in Indo-European roots.]ac?id·ly adv.ac?id·ness n.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.acid (?æs?d) n1. (Chemistry) any substance that dissociates in water to yield a sour corrosive solution containing hydrogen ions, having a pH of less than 7, and turning litmus red. See also Lewis acid2. a sour-tasting substance3. (Recreational Drugs) a slang name for LSDadj4. (Chemistry) chem a. of, derived from, or containing acid: an acid radical. b. being or having the properties of an acid: sodium bicarbonate is an acid salt. 5. sharp or sour in taste6. cutting, sharp, or hurtful in speech, manner, etc; vitriolic; caustic7. (Environmental Science) (of rain, snow, etc) containing pollutant acids in solution8. (Geological Science) (of igneous rocks) having a silica content of more than 60% of the total and containing at least one tenth quartz9. (Metallurgy) metallurgy of or made by a process in which the furnace or converter is lined with an acid material: acid steel. [C17: (first used by Francis Bacon): from French acide or Latin acidus, from ac?re to be sour or sharp] ?acidly adv ?acidness n ?acidy adjCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014ac?id (?æs ?d) n. 1. a compound usu. having a sour taste and capable of neutralizing alkalis and turning blue litmus paper red, 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. 2. a substance with a sour taste. 3. biting criticism or sarcasm. 4. Slang. the drug LSD. adj. 5. a. belonging or pertaining to acids or the anhydrides of acids. b. having only a part of the hydrogen of an acid replaced by a metal or its equivalent: an acid phosphate. c. having a pH value of less than 7. Compare alkaline. 6. characterized by a high concentration of acid. 7. sharp or biting to the taste; sour: acid fruits. 8. sharp, biting, or ill-natured in mood or manner; caustic: acid wit. 9. vividly intense in color: acid green. 10. (of igneous rock) rich in silica. [1620?30;