Question: 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.)

Answer: 
Allow me to disagree with @xLeitix.

No. It's a conference paper.
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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](http://www.siggraph.org/), 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](http://www.acm.org/publications/policies/conference-proceedings-in-acm-journals). (SIGGRAPH's practice is grandfathered.) But this doesn't stop non-ACM conferences like [VLDB](http://www.informatik.uni-trier.de/~LEY/db/conf/vldb/index.html) from wearing similar camouflage.
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](http://www.vldb.org), 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?