A heating element was a device that radiated heat for the purpose of warming a surface or room.
The oracle room on Yonada made use of heating elements to punish blasphemers. In 2268, Spock successfully neutralized them before they could increase the heat beyond liveable temperatures. (TOS: "For the World is Hollow and I Have Touched the Sky")
In 2369, the Arkarian waiter blamed Geordi La Forge's concerning energy readings on a malfunctioning heating element underneath the table. When the engineer offered to fix it, Orton told him not to worry about it. (TNG: "Starship Mine")