I'd always thought they were completely the same, but I saw a twitter thread in which this was questioned.
I mean here "working paper" only in its scholarly sense - I'm aware that the term has other uses, for instance at the UN where it can mean something akin to the draft of a resolution.
If the answer is that there is a meaningful difference in some disciplines but not in others, which disciplines treat the term differently?
As per the other (current) answer, in fields I know (pure and applied math, computing, GIS, ...) a working paper is less mature than a preprint. However, there is not necessarily a progression from one to the other.
The expectation is that a preprint is basically ready for submission to peer-reviewed journal or other venue, and may even already be submitted by the time anyone else reads it. In some cases, that may not be strictly true: I've seen working papers where there was a specific clear gap or two that still needed to be filled before submission, and I've myself had to rework preprints significantly based on friendly feedback received and/or different format/length expectations where ultimately submitted.
In fields I know, "working paper" carries connotations of work-in-progress. It may still be under active development, or paused to be revisited much later, or maybe never. But someone feels it is worth sharing and having in citable form regardless of any half-bakedness. Nothing wrong with that, and during my Ph.D. I was strongly influenced by a working paper that outlined an approach the author never got around to completing.
A preliminary scientific or technical paper. Often, authors will release working papers to share ideas about a topic or to elicit feedback before submitting to a peer reviewed conference or academic journal. Working papers are often the basis for related works, and may in themselves be cited by peer-review papers. They may be considered as grey literature.
From the same source:
Sometimes the term working paper is used synonymously as technical report. Working papers are typically hosted on websites, belonging either to the author or the author's affiliated institution.
My take: A preprint (or technical report) is more mature than a working paper.
