major axisn. The longer of the two lines about which an ellipse is symmetrical; the axis that passes through both focuses of an ellipse.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.major axis n (Mathematics) the longer or longest axis of an ellipse or ellipsoid Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014ma?jor ax?is n. the axis of an ellipse that passes through the two foci. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.