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.
a. Of, relating to, or containing an acid.
b. Having a high concentration of acid.
c. Having the characteristics of an acid.
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 acēre, to be sour; see ak- in the Appendix of Indo-European roots.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2019 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.