In hvac what's the difference MBH and BTU

BTU = British Thermal Unit = amount of energy needed to heat 1 pound of pure water by one degfree fahrenheit.

MBH = 1000 BTU's per hour. Note this is a measure of energy per hour, or power. My guess as to why the first letter of this term is "M" (instead of the more logical "K") is that whoever invented this term may have borrowed the "M" from Roman Numerals (M = 1000).