What does the UK and Australia consider Shakespeare's "Macbeth" to be?

That's the concept, regardless of whether they (we) chose to name it differently.

> A lot of non-techies, or techies with not much exposure to the Linux/xBSD world, consider "PD" -- public domain -- to be more or less the same as FOSS.

I find a lot of old "public domain software" – especially from the 1970s and (early) 1980s, it seems unclear what exactly the authors mean by "public domain". A few even assert it is both "public domain" and "copyright" simultaneously, suggesting they actually think "public domain" means "freely redistributable" or something like that. But, nowadays understanding of the difference seems much more common. Most FOSS licenses come with conditions – MIT/BSD are often presented as "liberal", but even they have attribution requirements. PD has no conditions, including no legal requirement for attribution. If you can't legally pass it off as your own work, it isn't PD. A zero-condition copyright license, like so-called "0BSD", is practically equivalent to PD though, even if the work technically remains copyrighted.

I agree that it is unethical to pass someone else's work off on your own. But legal attribution requirements cause a lot of bureaucratic pain if you worry about technical compliance with them. I don't like that pain and feel bad about imposing it on others, which is why for personal projects I prefer PD or PD-equivalent like 0BSD. I think attribution is best left as a matter of legally-unenforceable ethics.

> PD, BTW, is a USA concept and doesn't really exist in the rest of the Anglosphere.

Public Domain absolutely does exist in English law, and the rest of the Anglosphere. Copyright exists for a limited time (usually 70 years after death of author, but details vary); when it expires, the work enters the public domain.

I think what you are alluding to though, is there is a legal dispute about whether you can voluntarily put a work into the public domain by relinquishing the copyright before it expires. It is pretty clear under US law you can; it is very debated under non-US law. In some countries, the consensus answer seems to be "No" (for example, Germany). From what I understand, in the UK/Australia/etc, the question is totally unsettled, different lawyers have different opinions and no court has ever ruled on it.

However, in practice, it doesn't make a huge difference. Most people nowadays who try to put things in the public domain use something like Unlicense or CC0 – which basically contain a super-liberal fallback copyright license. So, if any court rules that the attempt to explicitly put the work in the public domain is void, the fallback copyright license means "it is still technically copyrighted but you can treat it as if you were public domain"

> suggesting they actually think "public domain" means "freely redistributable" or something like that

I definitely recall some people understanding "public domain" in that sense in the 1980s. Other people, frustrated with that understanding, did try to encourage adoption of "freeware" for "freely redistributable" software that was still subject to copyright. I don't recall seeing a great deal of distinction between the two prior to the publication of "The Cathedral and the Bazaar".

> I find a lot of old “public domain software” – especially from the 1970s and (early) 1980s, it seems unclear what exactly the authors mean by “public domain”. A few even assert it is both “public domain” and “copyright” simultaneously,

In the US, works that are subject to copyright are copyrighted by operation of law at the point of creation (there is basically no dispute about this.) There is (and this was more popularly adhered to, whether actually more popular or just more relevant, before F/OSS licensing became popular) a theory that the copyright owner can dedicate a work to the public domain (though, even among those who adhere to it, there has never been a single consensus on the mechanics of doing so, and some US copyright experts disagree that it is possible or claim that if it is, it is difficult [0]), so it is certainly plausible that someone would think it would be useful to assert that they were the copyright owner when also stating that the work had been dedicated to the public domain.

[0] Most, however, who think that public domain dedication as usually done doesn’t actually legally put a work in the public domain in US law do think it makes it harder for the copyright owner to later legally enforce the copyright, because of various other legal principles, like promissory estoppel.