"Shadow libraries" are also known by many other names; sometimes pirate libraries, extra-legal libraries, or just simply, libraries. "Shadow" can be a verb, as in "to shadow" something, in this case libraries that operate within the copyright domain. They usually consist of textual reading material, but also include audiovisual media. Some examples of popular academic shadow libraries are Library Genesis and Sci-Hub.
"Shadow" can also be interpreted that they exist "in the shadows", in obscurity; a wide variety of methods is often used to limit access to their collections, including paywalls, http-passwords, word-of-mouth, sneakernet and user registration.