カイロ 懐炉
A device that generates heat and is used to keep the hands warm.  These devices can be disposable or reusable and they produce heat through various means, such as chemical reaction, burning fuel, or using electricity.

Disposable hand warmers are a simple sealed pouch filled with material that undergoes a chemical reaction that produces heat.  Reusable hand warmers commonly produce heat either through the combustion of a fuel, as is the case with charcoal or lighter fluid hand warmers, or using electricity through a heating coil, as is the case for battery hand warmers.

A device similar to a disposable hand warmer that has been designed to adhere to the bottom of a sock is referred to as a [[toe warmer]].
hand warmer