TDY stands for Temporary Duty, which is a designation reflecting a United States Armed Forces service members or civilian Department of Defense employees travel or other assignment at a location other than their permanent duty station as authorized by the Joint Travel Regulations. TDY assignments are typically short-term, usually from two to 189 days in length. While the U.S. Army and Air Force use TDY, the U.S. Navy and Marines use TAD, and other federal agencies have no official designation for this kind of assignment.

TDY assignments can be for a variety of purposes, such as training, special assignments, or deployments. Some locations have furnished apartments for long-term stays, which have fully equipped kitchens so TDY recipients have the option to cook rather than always eat out, and some may have free washing machines and clothes dryers. Some government agencies consider any assignment over 45 days as an extended TDA, which allows the employee to be reimbursed for part of the expenses before the end of the assignment.

TDY orders are outlined in travel orders, which contain authorized entitlements such as transportation mode, duration of TDY, lodging, meal rate, and any other authorized miscellaneous expenses. The financial characteristics of TDY are perhaps the most important piece to understand, as TDY orders offer the most flexibility for servicemembers and their dependents to determine how they will handle assignments, placing them in a location anywhere from just a few days to six months.