di·phen·hy·dra·mine (d??f?n-h??dr?-m?n?, -m?n)n. A sedating antihistamine drug, C17H21NO, used usually in the form of its hydrochloride to treat symptoms of colds and allergic conditions, motion sickness, insomnia, and parkinsonism.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.diphenhydramine (?da?f?n?ha?dr??mi?n) na compound used in medicines, in the form of diphenhydramine hydrochloride, as an antihistamine, sedative, and anticholinergic Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014