Source : Webster's Revised Unabridged Dictionary (1913)

Obsolescent \Ob`so*les"cent\, a. [L. obsolescens, -entis, p. pr.
   of obsolescere, to wear out gradually, to fall into disuse;
   ob (see {Ob-}) + solere to use, be wont.]
   Going out of use; becoming obsolete; passing into desuetude.

Source : WordNet®

obsolescent
     adj : becoming obsolete