What does depreciation expense mean?

What does depreciation expense mean?
It is assumed that long lived tangible assets reduce in value over time. This reduction in value is hard to quantify economically, but is an acceptable reduction in income when calculating income tax. This reduction in value due to use or age of a long lived asset is called depreciation. The reduction or offset against income is called depreciation expense. Usually this is not an expense that requires the immediate expenditure of cash, but is called a non-cash expense deducted from income before calculating income tax. Generally the depreciation expense amounts are calculated from formulas promulgated by tax regulators to either model actual economic depreciation, or to motivate certain economic behaviors by allowing favorable tax treatment for the favored activities.