An expense is the reduction in value of an asset as it is used to generate revenue. If the underlying asset is to be used over a long period of time, the expense takes the form of depreciation, and is charged ratably over the useful life of the asset. If the expense is for an immediately consumed item, such as a salary, then it is usually charged to expense as incurred. If an expenditure is for a minor amount that may not be consumed for a long period of time, it is usually charged to expense at once, to eliminate the accounting staff time that would otherwise be required to track it as an asset.