From the Document: "This report provides data on expected departures and overstays, by country, for foreign travelers to the United States who entered as nonimmigrants through an air or sea port of entry (POE) and who were expected to depart in FY 2018 (October 1, 2017 - September 30, 2018). It does this by examining the number of entries, by country, for foreign travelers who arrived as nonimmigrants during this time as of October 1, 2018. An overstay is a nonimmigrant who was lawfully admitted to the United States for an authorized period, but remained in the United States beyond his or her authorized period of admission. The authorized admission period 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. [...] DHS has engaged in a concerted campaign to end visa overstay abuse. For the second year in a row, visa overstay rates have declined. DHS will continue efforts to ensure the integrity of its nonimmigrant visa program."
Department of Homeland Security: https://www.dhs.gov/