Learn in the textbookSoftware Distribution and Usage Models
DefinitionPublic domain software (パブリックドメインソフトウェア) is software whose copyright has been waived or has expired at the end of its protection period, so anyone may use it without permission.
ExplanationSince no one can claim rights over it, using, copying, modifying and redistributing it need neither permission nor payment. You may even build a modified version into your own product and distribute it.
AnalogyIt is like a public square that belongs to no one. Using it needs neither permission nor a fee.
Memory hookPublic domain means "the public's territory": the work has left any individual's hands and belongs to everyone.