The systematic allocation of the cost of an asset from the balance sheet to Depreciation Expense on the income statement over the useful life of the asset. (The depreciation journal entry includes a debit to Depreciation Expense and a credit to Accumulated Depreciation, a contra asset account). The purpose is to allocate the cost to expense in order to comply with the matching principle. It is not intended to be a valuation process. In other words, the amount allocated to expense is not indicative of the economic value being consumed. Similarly, the amount not yet allocated is not an indication of its current market value.
To learn more, see Explanation of Depreciation.