outdoor unit (of an air conditioner or heat pump)
Refers specifically to the external component of a split-system air conditioner or heat pump, containing the compressor and condenser.
エアコンの室外機がうるさい。
The outdoor unit of the air conditioner is noisy.
Please do not put things on top of the outdoor unit.
Compound of 室外 (shitsugai, 'outdoors') + 機 (ki, 'machine, device').