Abbreviation of terminate and stay resident. Refers to DOS programs that can be memory resident (remaining in memory at all times once they are loaded). Calendars, calculators, spell checkers, thesauruses, and notepads are often set up as TSRs so that you can instantly access them from within another program. TSRs are sometimes called pop-up programs because they can pop up in applications.
When you install a TSR, you define a special key sequence (usually a control character) that will invoke the TSR program. You can then press this hot key from within any application to run the TSR program. Many programs can be installed as a TSR, but TSRs reduce the amount of memory available to other programs. In addition, not all TSRs interact well with each other. You may have difficulties, therefore, if you try to keep too many TSRs in main memory at once.