A software module that extends or enhances the capabilities of an existing application.

synonym: plug-in.

Dutch: insteekkaart, uitbreidingskaart, kaart
Polish: karta, wtyczka, plug-in, plugin

A component that is added to something to improve it.

synonyms: addition, improver.

A supplementary component that improves capability.

synonyms: accessory, appurtenance, supplement.

Dutch: hulptoestel
Polish: akcesorium, dopełnienie, dodatek, uzupełnienie