Breadcrumb Home > Thesaurus > P Synonyms of principal diagonal Find synonyms for: Noun1. main diagonal, principal diagonal, diagonalusage: the diagonal of a square matrix running from the upper left entry to the lower right entryWordNet 3.0 Copyright © 2006 by Princeton University. All rights reserved. ✖ Related Content Daily Word Quiz: legerdemain Analogy of the Day: Today’s Analogy Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language