Rheostat
Part of speech: noun
Definition: An electrical resistor, with two terminals, whose resistance is continuously variable by moving a knob or slider.
Dimmer
Part of speech: adjective
Definition: Not bright or colorful.
Part of speech: noun
Definition: A rheostat that is used to vary the intensity of a domestic electric lightA switch used to select between the low and high headlamp beam on a road vehicle. (usually as "dimmer switch", primarily in North America; elsewhere "dipswitch" or "dipper switch")