dimmer switch

/D IH1 M ER0 S W IH0 CH/
noun
  1. 1

    A light switch which can adjust the brightness of the switched lamp from dim to bright, as well as turning it on and off.

  2. 2

    A switch that changes the headlamps on a motor vehicle from low to high and from high to low beam; a high-beam switch; a brights switch.

Source: https://en.wiktionary.org/wiki/dimmer%20switch