Christmas diseasen. A type of hemophilia that is caused by a deficiency of factor IX.[After Stephen Christmas, 20th-century British boy who was first diagnosed with it.]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.Christmas disease n (Pathology) a relatively mild type of haemophilia, caused by lack of a protein (Christmas factor) implicated in the process of blood clotting [C20: named after S. Christmas, the first patient suffering from the disease who was examined in detail]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014