Depreciation refers to the gradual and permanent decrease in the value of fixed assets over time due to causes like wear and tear, depletion, obsolescence, and accidents. It is calculated through methods like straight line and diminishing balance that allocate the cost of an asset over its useful life. Recording depreciation through adjusting entries debits the depreciation expense account and credits the asset account, while the closing entry reverses this to debit profit and loss and credit the accumulated depreciation account. The objective is to ascertain accurate profits, costs, and asset values.