Definition
A BTU (British Thermal Unit) is the amount of energy needed to raise one pound of water by one degree Fahrenheit. In HVAC it describes capacity: how much heating or cooling a system can deliver. Furnaces are rated in BTUs of heat output per hour; air conditioners and heat pumps are often expressed in tons, where one ton equals 12,000 BTUs per hour.
Why this matters for your home
More BTUs isn't automatically better. An oversized system short-cycles, wastes energy, and leaves humidity behind; an undersized one runs constantly and still can't keep up on a design-cold January night. The right BTU capacity comes from a load calculation, not a rule of thumb.