The Collaborative International Dictionary of English v.0.48:

Phase out \Phase" out`\ (f[=a]z" out`), v. t.
   To halt (an activity) in steps, or gradually; as, to phase
   out use of 386-based PC's.
   [PJC]

WordNet (r) 3.0 (2006):

phase out
    v 1: terminate gradually [ant: phase in]