What is Depletion?

Legal Definition
Depletion is an accounting concept used most often in mining, timber, petroleum, or other similar industries. The depletion deduction allows an owner or operator to account for the reduction of a product's reserves. Depletion is similar to depreciation in that it is a cost recovery system for accounting and tax reporting. For tax purposes, the two types of depletion are cost depletion and percentage depletion.

For mineral property, one generally must use the method that gives you the larger deduction. For standing timber, one must use cost depletion.

According to the IRS Newswire, over 50 percent of oil and gas extraction businesses use cost depletion to figure their depletion deduction. Mineral property includes oil and gas wells, mines, and other natural deposits (including geothermal deposits). For that purpose, property is each separate interest businesses own in each mineral deposit in each separate tract or parcel of land. Businesses can treat two or more separate interests as one property or as separate properties.
-- Wikipedia
Legal Definition
To take from the land's value when minerals are taken.