olemus
lisandprogramm, mis on hõlpsalt installitav ja
laiendab senise programmi võimalusi
= a software add-on that is installed on a program, enhancing its capabilities

ülevaateid
https://softwareengineering.stackexchange.com/questions/316199/differences-between-the-terms-modules-plugins-extensions

https://en.wikipedia.org/wiki/Plug-in_(computing)