The process of making or becoming better
betterment, amelioration, melioration, rectification, change, amendment, alteration, reformation, progression, advance, advancement, development, refinement, growth, rise, reclamation, civilization, gain, cultivation, progressiveness, increase, enrichment, promotion, recovery, recuperation, healing, regeneration, furtherance, renovation, modernization, remodeling, refurbishment, repair, reorganization, reform, emendation, revision, elaboration, enhancement, elevation, preferment, ennoblement, shaking down*, getting the bugs out*; see also revision.
Antonyms decay*, deterioration, retrogression.
That which has been improved
addition, supplement, repair, extra, attachment, correction, reform, remodeling, modernization, betterment, refinement, luxury, advance, advancement, step forward, enhancement, new model, deluxe model, latest thing*, last word*; see also change 2.