In the U.S. military, the abbreviation "TDY" stands for temporary duty. Temporary duty refers to the situation when a soldier or other employee is assigned to serve at a location that is not his permanent station.
Generally, TDY assignments are shorter than six months in duration. If an assignment lasts longer than that, most organizations consider the assignment to be a permanent change of station. Soldiers can be assigned anywhere for their TDY assignments.
During their TDY, many employees receive per diem pay. They usually do not have to pay for their food or lodging as the group responsible for the assignment covers those costs.