ASCUS: An acronym for Atypical Squamous Cells of Undetermined Significance. This term is used in the Bethesda System for reporting Pap smear findings and indicates that some flat (squamous) cells look unusual and may or may not be pre-malignant or malignant. Of all Pap tests with an ASCUS reading, 90-95% are caused by benign conditions, chiefly infections. The remaining 5-10% prove to be precancer or cancer.