It's called a Pavlovian response, or Pavlovian conditioning (named for the man who discovered it, a guy named Ivan Pavlov). See the Related Links below for more information.

Pavlovian response.

learned behavior

Pavlovian response

An automatic reaction is a synonym for absence of thought, gut reaction, or a knee-jerk reaction. It can also be thought of as an involuntary impulse or a Pavlovian response.

Pavlovian behaviour is an example of indoctrinated response.

The original experiment had dogs being fed while ringing a bell. After a while the dogs would start drooling in anticipation of food at the sound of the bell, even if they didn't get any food - so strong was their association of the sound and the meal.