Refrigerant
The fluid that carries heat out of your home in an AC or heat pump.
Refrigerant is the special fluid that cycles between the indoor and outdoor coils, absorbing heat inside and releasing it outside. It isn't 'used up' like fuel — a system that's low on refrigerant has a leak, and simply topping it off without finding the leak is a temporary patch. Low refrigerant shows up as weak cooling, a frozen coil, or hissing. Handling it requires a licensed technician.