A user agent is some combination of hardware and/or software used to render the content of a publication (i.e., literally the agent users use to consume content).

The term reading system is more commonly used with EPUB, but a reading system is just a more specific name for a user agent for digital publications.

The nature of any user agent is highly dependent on the context in which the term is used (e.g., web specifications typically use the term user agent to refer to browsers).