1. The process of falling into disuse or becoming out of date: Obsolescence frequently takes place because a replacement has become available which is more advantages compared to the disadvantages incurred by using or repairing the original system.
2. A loss in the utility or value of property that results over time from intrinsic limitations; such as, outmoded facilities or external circumstances: Obsolescence is usually distinguished from depreciation and physical deterioration.

Economic obsolescence results from external factors; such as, location that renders a property of no longer being competitive, or is unattractive to purchasers or investors, or of decreasing usefulness.

Functional obsolescence derives from a lack of adequate or appropriate equipment, space, or design.

3. In medicine, the termination or the beginning of the end of any physiological process.
4. Synonyms of obsolescence: on the way out, passing out of use, disappearing, fading, waning, on the wane, dying.