Michaelmas daisyn. Any of several asters that have leafy stems and flower in the fall, especially the New York aster.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.Michaelmas daisy n (Plants) Brit any of various plants of the genus Aster that have small autumn-blooming purple, pink, or white flowers: family Asteraceae (composites) Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014Mich?aelmas dai?sy n. an aster. [1775?85] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.