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(PD) The total absence of copyright protection. If something is "in the public domain" then anyone can copy it or use it in any way they wish. The author has none of the exclusive rights which apply to a copyright work.
The phrase "public domain" is often used incorrectly to refer to freeware or shareware (software which is copyrighted but is distributed without (advance) payment). Public domain means no copyright -- no exclusive rights. In fact the phrase "public domain" has no legal status at all in the UK.
See also archive site, careware, charityware, copyleft, crippleware, guiltware, postcardware and -ware. Compare payware.