D layern. The lowest region of the ionosphere, existing only during the day and extending from about 40 to 65 kilometers (25 to 40 miles) above the earth. Also called D region.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.D layer n. the lowest region of the ionosphere, at an altitude of ab. 50 mi. (80 km). [1930?35] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.