Final answer:
Extensions can be best defined as tools for viewing and interacting with webpages. They enhance the functionality of web browsers and provide additional features or services.
Step-by-step explanation:
Extensions can be best defined as tools for viewing and interacting with webpages. They are small software programs that enhance the functionality of web browsers and provide additional features or services. For example, browser extensions like AdBlock Plus allow users to block advertisements on websites, while Grammarly helps users check their grammar and spelling when typing online.