Which term describes a program that extends the functionality of a host application?

The term that describes a program which extends the functionality of a host application is "plug-in." Plug-ins are designed to add specific features or capabilities to a larger software application without altering the core structure of that application. For example, browser plug-ins can provide additional functionality such as ad-blocking or enhanced multimedia playback, allowing users to customize their software experience.

Service packs refer to updates that provide improvements or fixes to software but do not typically add new features. Compatibility relates to how well different software or hardware work together, and an End User License Agreement (EULA) is a legal contract that grants the user permission to use the software under specific conditions. These terms serve different purposes and do not specifically denote a program that extends the functionality of a host application as a plug-in does.