Part 1

This is one of the most interesting questions that has appeared here lately, I have to say. The answer I can give you will not be definitive on academia/historians' views concerning the usage of the term Holocaust for events other than the Nazi genocide of Jews and so-called "gypsies", since there really isn't a complete consensus on the question but I can give you insights into some discussions surrounding the term and its usage, which I hope will be helpful.

Origin and Proliferation of the term Holocaust

So, for a bit of context: As many will be quick to point out, the term Holocaust has its origin in a Greek term holokauston, meaning "a completely (holos) burnt (kaustos) sacrificial offering" or generally burned sacrificial offering. While this is the etymological origin, the term had been used already in the 19th century to describe mass killings or massacres in association with fire. It had been used to describe e.g. the burning of alleged Christian heretics in France in a Church as well as, in the 1920s, to the mass slaughter of Armenians during WWI. Basically, the term as it had been in use in modern language developed in the 19th century had the strong connotation of mass-violence, and massacres, often associated with fire. It was not specifically created to describe the genocides of Jews by the Nazis, although before it became common as a shorthand for these events, people referred to "a holocaust" and not "the Holocaust" (note both the article and capitalization). As a quick aside, deniers will sometimes point to this and say "well, isn't that weird!" and the answer to that is, that, no, it isn't particularly. Words can change in their meaning and usage over time and this does not denote some nefarious agenda but a rather natural process as many a linguist will point out.

Anyways, while the term had been used after the war to refer to certain things the Nazis had done as "a holocaust", the term "the Holocaust" did emerge in the 1960s with historian Nora Levin's book The Holocaust: The Destruction of European Jewry, 1933-1945. Levin's book appeared around the same time as one of the standard works on the Holocaust to this day, Raul Hilberg's The Destruction of the European Jews and Levin does refer to some of the academic conventions in describing the events we know as the Holocaust that Hilberg and others before him, such as British historian Gerhard Reitlinger, used in a critical manner. Specifically, a problem Levin tries to address in her book is that historians such as Reitlinger, Hilberg and others framed the murder of the Jews of Europe, of course, as a genocide while at the same time using the language of its perpetrators to describe the events, specifically, they would write of the "final solution" or the "final solution of the Jewish Question" (Endlösung resp. Endlösung der Judenfrage in German) as a shorthand for these events. This is something Levin – rightly – criticized because it transported the Nazi world view in as much as Jews were stated to be a problem which required a solution in the form of murder. Hence, she opted for naming this collection of events the Holocaust as a new shorthand to describe them.

The term then started to catch on in public especially after the 1978 TV mini-series Holocaust starring Merryl Streep and James Woods. Interestingly enough, this TV mini-series is little remembered in terms of pop-cultural memory today but its impact on both the public awareness of the topic as well as the proliferation of the name Holocaust for the events of the Nazi genocide can't really be overstated. Airing twice on NBC (once in April 1978 and September 1979) in four episodes the nine and a half hour series reached about 220 million viewers in the United States. It was even more successful in West Germany in terms of reaching an audience: Airing in German translation in January 1979 it was watched by 15 million households with an estimate 20 million, or half of the West German population at the time, viewers. The only things more successful in West German television was one episode of the Schwarzwaldklink (a sort of German Dallas) and the Football (Soccer) European and World Champion games

There also was resistance to the term. Some historians such as Walter Laquer, who had relatives that perished in Nazi camps, as well as people in the religious Jewish community opposed the term Holocaust on the grounds of its original sacrificial and religious implications. They expressed that to them, the idea of Jews being murder in connection with a sacrifice for God was on offensive idea on many a level. For them, as well as other people, the term "Shoah" (Hebrew for "Catastrophe") is the preferred term to describe the genocide of Jews.

Others opposed the term on non-religious grounds. British historian Geoff Eley in a 1983 review for the heavily criticized book by Lucy Davidovitz on the Holocaust wrote in the London Review of Books that the term implied a "[a] certain mystification, an insistence on the uniquely Jewish character of the experience, is an inescapable aspect of this process." for which he heavily criticized its usage as for him, mystification was the opposite of the historian's goal.

Holocaust, Metanarratives, and Univerzalization

And this takes us to the next important issue: Eley makes a pertinent point about this specific as well as so many other terms. Seldom is a word merely a descriptor but it often carries with it a metanarrative, meaning a whole slew of assumptions and connections about causes, relationships, meanings, and importance. This is an essential feature of discourse and it is what motivated Levin to criticize her colleagues for the term "final solution" and is also present in other areas (just as a quick example: think about the different implications the terms "liberal" and "conservative" have in a European and US American context – this too is a perfect example of different metanarratives of these terms).

The metanarrative for the term Holocaust had undergone changes since it has entered popular usage and we can also trace different, and sometimes competing, metanarratives depending on the context the word is used. In 1983 when Eley wrote his essay and after the term had been established in popular consciousness only a few years, a metanarrative for the term Holocaust had already been established, both in academic as well as popular usage. One of the most common and the one that most historians prefer I describe in this answer regarding the Holocaust as a name for the persecution of Jews and so-called "gypsies": It is a term that acknowledges and contains the description of the structural and ideological differences when it came to the persecution of Jews and "gypsies" when compared to other persecuted groups (namely, both the infrastructure in place solely to kill one particular group as well as the ideological will to kill all members of the group world-wide), without attempting to moralize this difference or make any sort of statement, which was "worse", because when you deal in the category of Nazi atrocities against all its victims "worse" is not really a category that can cover it anymore.

While this is a preferred usage in academia today, it is far from the only one and not the only metanarrative attached to it. Another, let's call it popular rather than academic, metanarrative of the Holocaust was born from the long public ignorance of the specific Jewish experience of persecution in Western public and political discourse.