Definition
A unit of heat energy equal to the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit at sea level.
Plain English
A standard way to measure how much heat something gives off or absorbs. One BTU is roughly the heat from a single kitchen match burning all the way down.
Context Anchor
Seen in aviation maintenance discussions of heat, engine cooling, combustion, cabin heating, and air-conditioning system capacity.
Derivation
Called 'British' because the unit was developed under the British (imperial) measurement system, using pounds and degrees Fahrenheit rather than metric units. 'Thermal' comes from the Greek 'thermē' meaning heat. So literally: a heat unit measured in British units.
Why Pilots Care
BTU ratings help technicians choose and verify the correct heating or cooling capacity for aircraft systems.
Grounding Statement
Picture one pound of water: one BTU is roughly the heat needed to warm that water by one degree Fahrenheit.
Intuition Check
Do not read BTU as a temperature. Temperature tells how hot something is; BTU tells how much heat energy is involved.
Example Sentence 1
The cabin heater on this aircraft is rated at 45,000 BTU per hour.
Example Sentence 2
Before installing the new air conditioner, confirm its BTU output meets the aircraft's requirements.