Contactor
The electrical relay that switches power to your AC's compressor and fan when the thermostat calls for cooling.
A contactor is a heavy-duty switch inside the outdoor unit. When the thermostat calls for cooling, a small signal energizes the contactor, which closes and sends full power to the compressor and condenser fan. Over years of clicking on and off — and exposure to heat, dirt, and insects — the contacts pit, corrode, or stick. A failed contactor is a common reason an AC stays completely silent even though the thermostat is calling and the breaker is on. It's an inexpensive part, but replacing it means working with line-voltage wiring, so it's a job for an HVAC technician.