In a "list of publications" (e.g. for a PhD thesis), should a conference paper which is published as "proceedings" in a peer-reviewed journal be categorized as a "journal article"?
(I'm from Liquid Crystals field.)
Allow me to disagree with @xLeitix.
A paper qualifies as a "journal publication" if (and only if) it passes through the standard peer-review process for the journal in which the paper is published. If a paper is reviewed only by the program committee for a conference, it's a conference paper, even if the proceedings are published in a journal.
As a particularly confusing example, consider SIGGRAPH, the flagship conference in computer graphics. For many many years, SIGGRAPH has published its proceedings as a standard issue of the journal ACM Transactions on Graphics (aka "TOG").
SIGGRAPH accepts two types of papers: (1) direct submissions to the conference in response to the call for papers and (2) papers published in TOG in the last 12 months. Papers accepted from TOG are clearly journal publications, because they went through the standard TOG refereeing process. (Those papers are not republished in the proceedings issue of TOG, but they are listed in the "table of contents".) But direct submissions accepted by the program committee are not journal publications, because they did not go through the standard TOG refereeing process, even though they are published in TOG.
The distinction is all the more confused by the facts that in the graphics community, "SIGGRAPH paper" is a considerably more valuable label than "journal paper", in part because the reviewing process for SIGGRAPH is (or has been historically) considered more rigorous than the refereeing process for graphics journals.
Of course, the rest of the world doesn't share this preference—or perhaps more accurately, doesn't believe that anyone sane could have such a preference—which is precisely why SIGGRAPH publishes their proceedings in a journal. Put bluntly, the graphics community chose to defend itself against lazy scholars who value the imprimatur of a journal above the quality and impact of the work through deception. Or if you prefer a slightly less inflammatory term: camouflage.
It's worth noting that ACM now explicitly forbids conferences from publishing proceedings in an ACM journal. (SIGGRAPH's practice is grandfathered.) But this doesn't stop non-ACM conferences like VLDB from wearing similar camouflage.
Unless I'm missing something here, you mean the conference proceedings were published in the conference series of some journal. If this is the case, what is published is certainly peer-reviewed, but is certainly not a journal publication. A classic example in my field (Physics) is the Journal of Physics G: Nuclear and Particle Physics. But there is a clear line of demarcation here - whenever the proceedings of any conference get published with J. Phys., they are in the Journal of Physics :Conference Proceedings. This leaves no ambiguity - what is published in the latter in certainly a conference proceeding and can't be classified as a journal article. (Actually, articles in the latter carry that identifier. e.g. have a look at this one.)
I appreciate your concern - the only precaution that one has to take here is, not to mention an article in a manner that gives a wrong impression. This is because the rigor of reviewing process is different for a journal article and a conference proceeding (in general). For example, if the same article gets loosely mentioned as a ''Journal of Physics article'', it is inappropriate because it gives the impression that it is a journal article. That's why people divide generally divide into categories - journal articles, conference proceedings, conference talks, ''poster presented and abstract published'' sort of a thing.
But of course, if you clearly mention/ the same gets clearly mentioned via some identifier that the article in question is a conference article, there is no harm in putting it under the heading of peer-reviewed articles. It certainly is peer-reviewed.
Hope that helps.
As counter-intuitive as it sounds, there is no absolute rule for what is considered a "journal" and what is considered "conference proceedings". In most cases the distinction is trivial. Most journals are not associated with a specific event, and paper submission and review are entirely unrelated from any conference or meeting. Contrary, most events have dedicated proceedings, which contain all and only the accompanying papers to the presented talks.
However, there are curious cases where the line blurs. Some conference proceedings have self-styled themselves as "journals", presumably in an attempt to make their publications seem more important. A slightly different case is VLDB, which basically does not do actual peer-review of conference submissions anymore. Instead, you submit to the VLDB journal, your submission gets the traditional journal peer-review process, and once a year the authors of all papers accepted in this year for the journal are invited to present their paper at the annual meeting.
This is all to say that I think the "clear line of demarcation" isn't quite as clear as New_new_newbie says.
In a "list of publications" (e.g. for a PhD thesis), should a conference paper which is published as "proceedings" in a peer-reviewed journal be categorized as a "journal article"?
If it is called "proceedings", I would put it to the other conference papers. However, to be on the safe side, you can always just check how other authors of this conference are referring to their paper. Are they treating it as a conference talk, or a journal submission?
As long as you don't say anything that is not true, and do not go against your university's regulations, you can use the layout that you like the best.
In my thesis I categorized publications into:
In "Peer-reviewed articles" I included any papers, including proceedings, that had been peer reviewed (i.e. I received an anonymous referee report for them). Anyone interested in reading my thesis will be familiar enough with the journals to know which ones are a conference proceedings venue. Certainly my thesis examiners were.