The phrase "public domain" (PD) is commonly used to refer to works that are not protected by copyright law. Works in the public domain can be used freely, without obtaining permission from or compensating the copyright owner.
Works can enter the public domain in several ways:
It is important not to confuse public availability with public domain. Before using the work, check if the work is in the public domain. The availability of a work online has nothing to do with its copyright status.
Additionally, it is recommended, as a best practice to cite public domain works. Not citing your sources could be considered plagiarism, which can have severe professional and educational consequences.