Materials in the public domain can be used for any purpose and without having to seek permission for their use. There are three categories of materials not protected by copyright and therefore in the public domain:

1. Copyright ineligible: Facts and ideas are not protected by copyright. This may include laws of nature, mathematical expressions, names, ingredients, raw data, and more. 
2. Copyright expired: Copyright protection expires after a certain amount of time. In Canada, copyright generally lasts for the life of the creator plus 70 years after the year of death. At that point the creator’s works enter the public domain. Not all countries use the same 70 year term, but if you are using a work in Canada, you can apply Canadian copyright law. 
3. Copyright waived: Materials in which the copyright has been waived. This may be more difficult to assess. For example, all federal government created works in the United States are in the public domain, but all government works in Canada are protected under crown copyright. Individual creators may waive their copyright and indicate this through a public domain, or CC0, status, provided by Creative Commons.