Compressor
The pump at the heart of an AC, heat pump, or fridge — the expensive part.
The compressor is the motor-driven pump that pressurizes refrigerant and drives the whole cooling cycle. It lives in the outdoor unit of an AC or heat pump, and inside a refrigerator. It's also the costliest component to replace — often enough to justify replacing the whole unit instead. Keeping coils clean and airflow open is how you protect it; running a system that's frozen or low on refrigerant is how you kill it.