thi·ol (th??ôl?, -?l?, -?l?)n. A sulfur-containing organic compound with the general formula RSH where R is any hydrocarbon group. Also called mercaptan.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.thiol (??a??l) n (Elements & Compounds) any of a class of sulphur-containing organic compounds with the formula RSH, where R is an organic group. Also called (not in technical usage): mercaptan Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014mer?cap?tan (m?r?kæp tæn) n. any of a class of odiferous sulfur-containing compounds having the type formula RSH, where R represents a radical. Also called thiol. [