Public domain software refers to computer programs that are not protected by copyright and are freely available for anyone to use, modify, and distribute. These types of software are often referred to as "open-source" or "freeware," as they serve as an alternative to proprietary software that requires purchased licenses. Other synonyms for public domain software include "shareware," which allows users to try the software before purchasing it, and "copyleft," which is a term used to describe software that is available for public use but requires any modifications or derivative works to also be public domain. Additionally, some software may be referred to as "GNU" or "GPL," which represents the GNU General Public License that governs the distribution and use of the software.