The term ‘browser extension’ refers to a small software module which is used to customise a web browser. Browsers typically allow for a variety of extensions, which user interface modifications, ad-blocking as well as cookie management.
Browser extensions are normally tasked with adding additional features as well as functionalities to a website. However these can also be used to remove features and functionalities that are unwanted, for example pop-up ads in addition to other aspects of a website’s core behaviour that a user wishes to opt-out of.
A browser extension must not be confused with a browser plug-in as these are a separate type of module. The main variation is that extensions are typically just source code however plug-ins are always executables (i.e. object code).
As of 2019, plug-ins have been deprecated by most browsers, while extensions are used widely. The most popular browser, which Google Chrome, has thousands of extensions available however only one plug-in: the Adobe Flash Player which is disabled by default.