Entry 1 sense Webster, 1913 Aphthous · Aph·thous · IPA 01 a. Pertaining to, or caused by, aphthæ; characterized by aphtæ; as, aphthous ulcers; aphthous fever. 1. Pertaining to, or caused by, aphthæ; characterized by aphtæ; as, aphthous ulcers; aphthous fever.