A BTU (British Thermal Unit) is the standard unit of heat energy used in HVAC systems in the United States. One BTU equals the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit.
In HVAC, BTUs are used to measure both heating and cooling capacity. Air conditioners, heat pumps, and furnaces are rated by how many BTUs per hour (BTU/hr) they can produce or remove. A 3-ton air conditioner, for example, has 36,000 BTU/hr of cooling capacity.
A useful rule of thumb: most homes require roughly 20-25 BTUs per square foot for cooling. However, accurate sizing always requires a Manual J load calculation that accounts for insulation, climate, windows, and occupancy.