Back-number is a term used to describe something that is old or outdated. Antonyms for this term would include new, current, modern, and up-to-date. These words are used to describe something that is relevant and timely, and not out of touch with current trends and ideas. In today's fast-paced world, it is important to stay current and informed, and using the right terminology to describe something is a key part of that. So, instead of referring to something as a back-number, using modern language and staying up-to-date can help keep the conversation relevant and engaging.