Definitions of Depreciation Accounting (AS) 6 :-

1. The following terms are used in this Standard with the meanings specified:
1.1 Depreciation is a measure of the wearing out, consumption or other loss of value of a depreciable asset arising from use, effluxion of time or obsolescence through technology and market changes. Depreciation is allocated so as to charge a fair proportion of the depreciable amount in each accounting period during the expected useful life of the asset. Depreciation includes amortisation of assets whose useful life is predetermined.
1.2 Depreciable assets are assets which
(i) are expected to be used during more than one accounting period; and
(ii) have a limited useful life; and
(iii) are held by an enterprise for use in the production or supply of goods and services, for rental to others, or for administrative purposes and not for the purpose of sale in the ordinary course of business.

1.3 Useful life is either (i) the period over which a depreciable asset is expected to be used by the enterprise; or (ii) the number of production or similar units expected to be obtained from the use of the asset by the enterprise.

1.4 Depreciable amount of a depreciable asset is its historical cost, or other amount substituted for historical cost1 in the financial statements, less the estimated residual value.