Public domain refers to creative materials that are not protected by copyright, either because their copyright term has lapsed or because they were never subject to copyright in the first place.
Although public domain is an important topic to consider in order to understand copyright law, it is not actually a part of copyright law! Rather, anything that is not covered in copyright law can be considered to be part of the public domain.
Everyday year on January 1, works that had previously been protected by copyright enter the public domain, leading to some to refer to January 1 as "Public Domain Day!"
Image credit: "happy public domain" by Laurel L. Russwurm is marked with CC0 1.0.
