In accounting, accretion expense is the expense created when updating the present value

Present value

Present value, also known as present discounted value, is the value on a given date of a future payment or series of future payments, discounted to reflect the time value of money and other factors such as investment risk...

(PV) of a balance sheet item.

For example, if one originally recognizes the present value of a liability at $650, which has a future value

Future value

Future value is the value of an asset at a specific date. It measures the nominal future sum of money that a given sum of money is "worth" at a specified time in the future assuming a certain interest rate, or more generally, rate of return; it is the present value multiplied by the accumulation...

(FV) of $1000, every year one must increase the PV of the liability as it comes closer to its FV. If the above liability, for example an asset retirement obligation, had a discount rate of 10%, accretion expense in year 1 would be $65, and the PV of the liability at the end of year 1 would be $715.