A plugin (sometimes written as plug-in) is a software program or a collection of software files that can be added or installed to an application to provide additional functionality to that application.
In WordPress, plugins allow you to easily modify, customise, and enhance a WordPress site. Instead of changing the core program code of WordPress, you can add functionality with WordPress Plugins. Here is a basic definition.
A WordPress Plugin is a program or a set of one or more functions written in the PHP scripting language, that adds a specific set of features or services to the WordPress site. You can seamlessly integrate a plugin with the site using access points and methods provided by the WordPress Plugin Application Program Interface (API).