MMBtu
One million British thermal units — the energy-content unit natural gas is often priced on, including the Henry Hub benchmark.
MMBtu means one million British thermal units, a measure of energy rather than volume. A Btu is the heat needed to raise one pound of water one degree Fahrenheit, and "MM" is the convention for a million. Natural gas is frequently priced per MMBtu — the widely quoted Henry Hub benchmark is an MMBtu price.
Gas volume in Mcf converts to MMBtu through the gas's heating value. A common rule of thumb is that one Mcf is roughly one MMBtu, but the exact factor depends on composition; richer gas runs above 1.0 and lean gas can run below. Your settlement statement may quote either unit, so it pays to know which one a price refers to.
For a mineral owner, the practical point is that the MMBtu price, the volume produced, any post-production deductions, and your decimal interest together determine the gas portion of your royalty check.