I saw a job posting that says
PhD or equivalent required.
What is meant by equivalent? What can be an equivalent for PhD? Expertise gained through work experience and publications?
It could also be an equivalent degree from another country. For example, a degree, equivalent of PhD, in Russia is called 'Candidate of science' (https://en.wikipedia.org/wiki/Education_in_Russia)
In an advertisement for a tenure-track position (assuming from the tags on your question), in the absence of more specific information, I would assume this wording is meant to include other names for a doctoral degree, e.g. DPhil or one of the other degrees on this list.
Sometimes it even specifically says as much, e.g. here:
PhD or equivalent biomedical doctorate(s)
Presumably this wording also allows them to hire someone without a PhD, without having HR make a huge fuss, but I would assume that would be a rare situation.
If it said "or equivalent experience," then perhaps it could be interpreted to mean equivalent experience gained outside of a doctoral program.
While it does seem to be less common now relative to several decades ago, I certainly know, and know of, several folks who had masters degrees, went to work at US National Labs, and were world leaders in their technical fields. Often, to their amusement, they were introduced at conferences as "Dr. So-and-so". None that I knew personally moved to university positions, but most had been contacted about their interest in doing so. All were quite capable of being a professor at a research-oriented university, with strong publication records and funding success.
A different question is what has changed since the, roughly, 1970's when this pattern seems to have been broken. One possibility is that there are many more graduate programs, so those kind of folks are probably just going on to get a PhD.
In addition to the answers given such as "Some doctorate degrees don't have the abreviation 'PhD'" that several people have provided, especially in the biomedical fields there may actually be several possible alternative degrees that might compete for the same position.
For example, there are positions in my field that might capable be filled by a PhD, and MD, or a DVM, each of whom would add their own particular twist to the position, but all of whom might serve the needs of a particular program.
PhD or equivalent. An experience equivalent to a PhD surely does exist and has been used by almost all major Universities a round the world in recruiting for tenured positions. Professional fields like Medicine, Law, where professional life are as rigorously academic as being in class still use it a great deal. A bachelor degree or masters degree holder but with relevant experience, could be considered an equivalent to a PhD. or better if he had comparable or better experience. However as the opportunities for PhD studies are increasing, most employers are today insisting on the actual PhD. However you will also realize that a lot of top level institutions look for the best brains not schooling history! I was once in recruiting an expert in Elephant Conservation. We had two great candidates. One had an M Sc in Conservation Biology with fifteen years experience working with elephants and 30 papers in top range refereed journals. The Other had a PhD in the same field, but with five years experience with elephants out of which he had published three papers in good journals. They were both very good, however the MSc fellow had greater academic depth than the PhD fellow. The MSc fellow was taken!
In some countries it is only literally called Doctor of Philosophy if it is a humanitarian / theoretical sciences subject. "Doctor of Technology" is another and "Doctor of Medicine" is a third, so the name kind of specifies the subject.
To start a PhD in some countries requires a MSc degree first and in some places a BSc is enough. In some areas (both geographically and by subject of study) one year worth of formal course work is enough in some two years is required. In some areas you are expected to produce maybe 5 or 6 peer-reviewed publications in some you may be able to get away with considerably less. Also the length of studies vary. From 3 years in some places to 5 in some and in practice often a bit longer than that.
There really exists no "equivalence". What they are polling for when phrasing it like that is probably not primarily any particular skill set, but rather a type of person who has the personality and mental drives fit for a particular type of role or position.
A Ph.D is a research degree which involves, in part, convincing a committee of professors that the student is capable of doing professional level research, and writing the results of such research in a formal style. No actual publication of the research is involved. An equivalent Ph.D could be actual publications in scientific journals without having attended, or completed, a formal university degree program.