Headless means that the application is running without a graphical user interface (GUI) and sometimes without user interface at all. There are similar terms for this, which are used in slightly different context and usage.

What does it mean for something to be headless?

headless means it runs without a graphical user interface (GUI). In most cases, headless applications are command line applications or applications that are interfaced from a programming language. ... This window is a graphical user interface (GUI).

What is a headless client?

Headless Client is used to offload AI calculations from the server instance. Headless Client is integrated into game client and dedicated server executable (Windows and Linux, use -client parameter)