/ Thesaurus / D Synonyms of d layer Find synonyms for: Noun1. D-layer, D regionusage: the lowest region of the ionosphere (35 to 50 miles up) that reflects low-frequency radio wavesWordNet 3.0 Copyright © 2006 by Princeton University. All rights reserved. Related Content Daily Word Quiz: blithesome Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language