You can waive your copyrights in which case you do not retain them. You can also, as you say, assign a public domain license and retain copyrights, but that isn't considered "public domain" per se.
'Examples for works actively dedicated into public domain by their authors are reference implementations of cryptographic algorithms,[6][7][8] NIH's ImageJ,[9] and the CIA's The World Factbook.[10] The term is not normally applied to situations where the creator of a work retains residual rights, in which case use of the work is referred to as "under license" or "with permission".'