Good old Wiki- the reference is posted below before the inane sneering begins
Obsolescence is the state of a being which occurs when an object, service or practice is no longer wanted even though it may still be in good working order. Obsolescence frequently occurs because a replacement has become available that is superior in one or more aspects. Obsolete refers to something that is already disused or discarded, or antiquated.
Fowler HW and Fowler FG (1st ed.), Thompson Delia (ed.) (1995).
The Concise Oxford Dictionary of Current English, 9th Ed., Clarendon Press, Oxford.