mi·fep·ri·stone (m?-f?p?r?-st?n?)n. A synthetic steroid, C29H35NO2, that blocks the action of progesterone and is used in combination with misoprostol to induce abortion in early pregnancy. Also called RU 486.[(a)mi(no) + alteration of phe(nyl) + pr(opyl) + alteration of est(radiol) + -one.]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.mifepristone (m??f?pr??st??n) n (Pharmacology) an antiprogestogenic steroid, used in the medical termination of pregnancy. Formula: C29H35NO2[C20: from aminophenol + propyne + oestradiol + -one]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014mif?e?pris?tone (?m?f ??pr?s to?n) n. an antigestational drug that prevents a fertilized egg from attaching to the uterine wall by blocking the action of progesterone. [1985?90; probably alter. of (a)mi(no)phe(nol) + pr(opyl) + est(radiol) + -one] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.