A public domain work is not protected by copyright and can be freely used by anyone.
Works fall under public domain if:
Is it Protected by Copyright?
Created by the ALA Office for Information Technology Policy, this digital slider tool helps to determine if a work is covered by copyright law or falls under the public domain.
Copyright Term and the Public Domain in the United States
This website from Cornell University Library helps to determine copyright status for various publication scenarios.
You can always check the copyright status of works through the Copyright Office's database.
These marks indicate that a work is within the public domain because copyright has expired or the author has intentionally placed the work in the public domain.