The length of time a non-citizen is permitted to remain in the United States under the terms of their visa or immigration status. This period is determined by the U.S. government and may be shown on a Form I-94 (Arrival/Departure Record), a visa stamp, or a formal notice of action from USCIS (such as an approval notice for an extension or change of status).
It’s important to understand that your visa expiration date is not the same as your period of authorized stay. The visa allows you to enter the U.S.; your I-94 or USCIS notice determines how long you can stay.