カイロ
A pad that serves to warm parts of the body. They may be disposable or re-usable.

A well-known brand in Japan is called <i>Cairo</i> (カイロ), after the Egyptian capital, to the point where the word is synonymous with <i>heat pad</i>; this is much like how <i>band-aid</i> has become a genericized trademark for <i>bandage</i> in the US.
heat pad