The core of EPANET should be designed to be lean and lightweight, to maximize flexibility and minimize code bloat. Plugins then offer custom functions and features so that each user can tailor EPANET to their specific needs.
A Plugin is a piece of software containing a group of functions that can be added to EPANET. They can extend functionality or add new features. EPANET Plugins are written in the Python programming language and integrate seamlessly with EPANET. They make it easy for users to add features to EPANET without knowing a single line of code
Adopted from https://codex.wordpress.org/Plugins and http://www.wpbeginner.com/glossary/plugin