plugin
Plugin (noun) means a separate piece of software that adds a feature to an existing program without changing the program itself, and which can be installed or removed independently. Example: “There is a plugin that adds a booking calendar to the site.”
How to use plugin
Learner’s notesIn plain EnglishAn optional extra you install to give a program a new feature.
Written plugin or plug-in; both are accepted, and plug-in is more usual in edited prose.
A plugin adds behaviour; a theme changes appearance. On many platforms they are installed the same way but do different jobs.
Trace the full origin ↓Fill the gap
Can you complete this real example?
There is a _____ that adds a booking calendar to the site.
Etymology
From the phrasal verb plug in, on the image of plugging an extra part into a socket.