noun

• (uncountable) The state of being obsolete—no longer in use; gone into disuse; disused or neglected.
  "Apart from this feature the Trichoptera also differ from the typical Neuroptera in the relatively simple, mostly longitudinal neuration of the wings, the absence or obsolescence of the mandibles and the semi-haustellate nature of the rest of the mouth-parts."
• (countable) The process of becoming obsolete, outmoded or out of date.
  "Shared obsolescences can be of some significance when connected with a functional replacement. If two languages replace an earlier word for 'nose' with some other term and also lose the traditional term, this double agreement is of significance for judging the existence of a common historical period."