Btu (15.8°C, ISO)
The British Thermal Unit (Btu) is a traditional unit of heat, defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. The Btu (15.8°C, ISO) is a specific variant standardized by the International Organization for Standardization (ISO) based on the calorie defined at 15.8°C.
Therm (US)
A US therm is 105,480,400 joules, which is 100,000 Btu (59°F) and roughly the energy in 100 cubic feet of natural gas. US gas bills are often given in therms.