Learner's definition of DAMPER
a flat piece of metal in a fireplace, furnace, etc., that controls the amount of air that can enter
a small piece of wood inside a piano that is covered with cloth and that stops the movement of a piano string
put a damper on
to make (something) less strong, active, or exciting
His bad mood put a damper on the celebration.
The rain put a damper on our plans for a picnic.
He was putting a bit of a damper on her spirits.