Definition of "handler"

handler

/ˈhæn.dlə/

noun
plural handlers
1. (literally) One who handles something (especially manually) or someone.
2. (in combination) A controller, trainer, someone who handles a specified thing, animal or person (especially a prizefighter).
  Quotations
  They are also very sympathetic, especially Caesar’s orangutan advisor, Maurice (Karin Konoval), who takes a shine to a human moppet (Amiah Miller) he finds hiding in the back of a shack, and the poignant Bad Ape (a scene-stealing Steve Zahn), a mangy chimp who was beaten so often in his zoo-animal days that he came to believe what his handlers were shouting was supposed to be his name.
  2017 July 7, Ignatiy Vishnevetsky, “The ambitious War For The Planet Of The Apes ends up surrendering to formula”, in The Onion AV Club‎
3. (in combination) An advisor or manager providing guidance to a person occupying public office or a prominent position.
4. (programming) A subroutine that handles a particular situation such as an event or exception.
  Quotations
  If the expression is a throw, we unwind the stack seeking a handler expression.
  2006, Hans-Wolfgang Loidl, Trends in Functional Programming, volume 5, page 62