The word "addon" refers to a software component that enhances or adds new features to an existing program. Some synonyms for addon include extension, plugin, module, add-in, and upgrade. An extension typically serves the purpose of improving the user interface or experience. A plugin and a module add further functionality to the application and are essential to support third-party applications, such as Adobe Flash Player. Add-ins are popular among business application users as they are designed to increase productivity by automating tasks within the app. Lastly, upgrades contain new features that are not already present in the installed version. Regardless of its name, addons are incredibly useful in complementing software and can make a significant difference in their functionality and user experience.