Entry 1 sense Webster, 1913 Phase out /(fāz" out`)/ · Phase· out · IPA /feɪ̯z äʊ̯t/ 01 v. t. To halt (an activity) in steps, or gradually; as, to phase out use of 386-based PC's. 1. To halt (an activity) in steps, or gradually; as, to phase out use of 386-based PC's.