Most back-number is a term used to describe something that is outdated or no longer relevant. There are many words that can be used as synonyms for this phrase, including antiquated, obsolete, outmoded, passe, old-fashioned, and retrograde. Antiquated refers to something that is so old-fashioned that it is no longer useful or effective. Obsolete suggests that something has been replaced by something more modern and efficient. Outmoded refers to something that is no longer fashionable or in style. Passe suggests that something is outdated because it has lost its appeal or relevance. Old-fashioned refers to something that is traditional or conservative in its style or approach. Retrograde indicates that something has gone backwards in terms of progress or development.