The term GAS-BILL UNIT is a unit of measurement used to calculate the cost of natural gas. The unit is represented by the word 'THERM', which is an acronym for one thousand cubic feet of natural gas. A therm is the amount of energy required to heat one thousand cubic feet of natural gas to a temperature of 100 degrees Fahrenheit. This unit of measurement is used to calculate the cost of natural gas for residential and commercial customers.