To stay in the United States longer than the period of time which Department of Homeland Security (DHS) gave to a person when he/she entered the United States, or to fail to meet the requirements or violate the terms of the visa classification. The person becomes “out of status.”
For example, you entered the U.S. on a student visa to study at a university. You work at your uncle's convenience store without authorization, and do not study. You have lost status. You are out of status.
Source: U.S. Department of State.