mo·daf·i·nil (m?-d?f??-n?l?)n. A nonamphetamine stimulant drug, C15H15NO2S, that promotes wakefulness and is used to treat narcolepsy and other sleep disorders.[moda-, origin unknown + alteration of (sul)finyl, one of its constituents.]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.modafinil (m???dæf?n?l) na stimulant drug used to treat narcolepsyCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014