Apps, Applications, Plug-ins, and Add-ons
In Confluence, these terms all mean the same thing. We prefer apps.
Applications (and it's abbreviation, apps), plugins, and add-ons are all interchangeable terms for any piece of software created to supplement or enhance Atlassian Confluence. All Brikit products are apps, and all Brikit products supplement, enhance, and beautify your Confluence experience. Apps can be downloaded from the Atlassian Marketplace.
To install, uninstall, enable, and disable apps, System Administrators can use the Universal Plugin Manager (UPM), a special interface built into Confluence that connects to the Atlassian Marketplace.