From the Executive Summary: "This report provides data on expected departures and overstays, by country, for foreign travelers to the United States who entered as nonimmigrants through air or sea ports of entry (POE) and who were expected to depart in FY 2020 (October 1, 2019 - September 30, 2020). An overstay is a nonimmigrant who was lawfully admitted to the United States but who remained in the United States beyond the authorized period of admission. The authorized period of admission can be a fixed period or for the duration of a certain activity, such as the period during which a student is pursuing a full course of study or any authorized technical/practical training. U.S. Customs and Border Protection (CBP) identifies two types of overstays: 1) individuals for whom no departure has been recorded (Suspected In-Country Overstays), and 2) individuals whose departure was recorded after their authorized period of admission expired (Out-of-Country Overstays)."
Department of Homeland Security: https://www.dhs.gov/