A more thorough explanation:

Definition: Makeup gas refers to natural gas that has been paid for by the purchaser under a take-or-pay contract but is to be delivered in the years following payment.

Example: A company enters into a take-or-pay contract with a natural gas supplier. The company pays for a certain amount of natural gas upfront, but the supplier is not able to deliver all of it immediately. The remaining gas that the supplier will deliver in the future is called makeup gas.

This example illustrates how makeup gas is a term used in the oil and gas industry to refer to natural gas that has been paid for but not yet delivered. It is a way for companies to secure a supply of natural gas for future use.