Public domain
The public domain is the pool of creative works that are not owned or controlled by anyone (anymore) - stuff that belongs to everyone.
Because in many jurisdictions intellectual property expires 70 years after death of the author lots of material gets in the publich domain this way.
Some projects have original content contributed entering the Public Domain unless the author explicitly states otherwise. This concept is called Primarily Public Domain (PPD), which has been extended to the concept of Ethical Public Domain.