Wikipedia
In accounting under the traditional historical cost paradigm, historical cost is the original nominal monetary value of an economic item. Historical cost is based on the stable measuring unit assumption. In some circumstances, assets and liabilities may be shown at their historical cost, as if there had been no change in value since the date of acquisition. The balance sheet value of the item may therefore differ from the real value.
While historical cost is criticised for its inaccuracy (deviation from real value), it remains in use in most accounting systems during low and high inflation and deflation. During hyperinflation, International Financial Reporting Standards require financial capital maintenance in units of constant purchasing power in terms of the monthly CPI as set out in IAS 29 Financial Reporting in Hyperinflationary Economies. Various corrections to historical cost are used, many of which require the use of management judgment and may be difficult to verify. The trend in most accounting standards is a move to more accurate reflection of the fair or market value, although the historical cost principle remains in use, particularly for assets of little importance.
Depreciation affects the carrying value of an asset on the balance sheet. The historical cost will equal the carrying value if there has been no change recorded in the value of the asset since acquisition. Improvements may be added to the cost basis of an asset.
Historical cost does not generally reflect current market valuation. Alternative measurement bases to the historical cost measurement basis, which may be applied for some types of assets for which market values are readily available, require that the carrying value of an asset (or liability) be updated to the market price ( mark-to-market valuation) or some other estimate of value that better approximates the real value. Accounting standards may also have different methods required or allowed (even for different types of balance sheet variable real value non-monetary assets or liabilities) as to how the resultant change in value of an asset or liability is recorded, as a part of income or as a direct change to shareholders' equity.
The Capital Maintenance in Units of Constant Purchasing Power model is an International Accounting Standards Board approved alternative basic accounting model to the traditional Historical Cost Accounting model.