V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

TSR
       Terminate and Stay Resident


The Free On-line Dictionary of Computing (30 December 2018):

Terminate and Stay Resident
TSR

   (TSR) A type of DOS utility which, once loaded, stays in
   memory and can be reactivated by pressing a certain
   combination of keys.