1. The planned or unplanned impairment, for example by aging or wear, of an asset.
2. Depreciation is the reduction in the value of assets. This impairment is due to wear and tear, changes in the market price or other factors of diminution.
http://economictimes.indiatimes.com/definition/depreciation (1.); http://www.gruenderhelden.de/definition-was-ist-eine-abschreibung/ (2.)