ac·yl (?s??l)n. Chemistry An organic group having the general formula RCO-, derived from an organic acid.[ac(id) + -yl.]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.acyl (?e?sa?l) n1. (Chemistry) (modifier) of, denoting, or containing the monovalent group of atoms RCO-, where R is an organic group: acyl group or radical; acyl substitution. 2. (Elements & Compounds) an organometallic compound in which a metal atom is directly bound to an acyl group[C20: from acid + -yl]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014ac?yl (?æs ?l, -il) n. the univalent group RCO?, where R is any organic group attached to one bond of the carbonyl group. [1895?1900;