Period over which an asset (machine, property, computer system, etc) is expected to be usable, with normal repairs and maintenance, for the purpose it was acquired, rented, or leased. Expressed usually in number of years, process cycles, or units produced, it is usually less than the asset's physical life, and is the period over which the asset's depreciation is charged. Also called average life, service life, effective life, mean life, or useful life.