(Terminate and Stay Resident) Refers to a program that remains in memory when the user exits it in order that it be immediately available at the press of a hotkey. TSRs were popular under DOS to quickly pop up a calendar, calculator or other utility, because DOS did not have built-in task switching. Standards for writing TSRs were not codified early on, and TSRs often conflicted with each other and regular applications. After Windows 3.0, TSRs became moot, since any DOS or Windows application could be conveniently task switched in a separate window.