What does it mean when a work is placed in the public domain?

When a work is placed in the public domain, it means that the author has relinquished the copyright on the work, or the copyright has expired due to other reasons such as the passage of time. This allows anyone to use, modify, copy, and distribute the work without any need for permission, and without the restrictions that copyright normally imposes.

Here’s a clarification of the other statements:

• The work was done by a government agency: While it is true that in some jurisdictions, like the United States, works created by federal government employees as part of their official duties are automatically in the public domain, this is not universally applicable and not the defining characteristic of public domain works.
• You must redistribute changes to the software: This condition is associated with certain open source licenses (particularly copyleft licenses), not with works in the public domain. Public domain works can be modified and redistributed without any obligation to share the changes.
• The author has died: The death of the author is one of the factors that can lead to a work entering the public domain, but it is not immediate. In most countries, copyright lasts for a certain number of years after the author’s death (commonly 70 years), after which the work enters the public domain.
• You may not use the work for commercial purposes: This is incorrect with respect to public domain works. Public domain status means there are no restrictions on the use of the work, including commercial use. Anyone can use public domain works for any purpose.

Thus, the correct interpretation of a work being in the public domain is that the copyright holder has relinquished their rights to the work, making it freely available for public use without restrictions.