Therm
A therm is 100,000 BTU of natural gas energy — the billing unit on US gas bills. Certified gas water heaters list annual use in therms per year, so multiplying by your local rate gives the yearly running cost.
The national average residential gas price is around $1.45 per therm; a typical certified gas storage unit using ~180 therms/year costs about $260 annually, while an efficient tankless at ~130 therms runs closer to $190.
The equivalent figures for other fuels: annual kWh for electric and heat pump units, and gallons per year for propane models.