The word originally hails from Greek, and has been used in English to refer to a burnt offering or sacrifice (usually in a Biblical context) from as early as the 13th century. From the 17th century on, there is evidence of it also meaning "a great slaughter, a massacre", especially by fire.

In English, according to the Oxford English dictionary, it was first used to refer to the extermination of the European Jews by the Nazis in December 1942 in a London newspaper.

However, for a full run-down of the issue I can do no better than refer you to this excellently footnoted and exhaustive effort by Sean Warsch in The Jewish Magazine of September 2006: A "holocaust" Becomes "the Holocaust".

The origin of the word Holocaust is the greek word holokauston which means "completely burned" or a burnt sacrifice for a god. It has since the 17th century been used as a term to describe the violent killing of larger numbers of people, but quite rarely - to me it seems like a word used in upper class writings to show off one's good education, including greek, quite like some scholars today still use latin vocabulary mid-sentence where it doesn't benefit the message (an example being in nuce).

On rare occasions, the word had also been used describing anti-judaistic pogroms in medieval times and in Italy the burning of witches was criticized as a "nova holocausta".

The development of the modern understanding of the word holocaust is closely linked with the history of the Armenians. Near the end of the 19th century they suffered under the Ottoman Empire. One instance where 1,500 Armenians were burned alive in a church was called a holocaust, and during the following years all of those violent eruptions of mass murder against Armenians were deemed part of "the Holocaust in Asia Minor". After World War I, Winston Churchill called the genocide of the Armenians an "administrative holocaust" which put the word into the meaning we still connect with it: genocide.

However, there still was some parallel use during the 30s and 40s - the November Pogroms were, in the old meaning, called a Holocaust as well as people in the late 40s called the bombing of Dresden a Holocaust because, well, thousands of people died in flames there.

However, the meaning shifted more and more towards describing only the genocide of the european Jews by Germany. Between roughly 1945 and 1960, it was mostly used as "the Holocaust of the european Jews" to describe this genocide. By 1960, especially with the Eichmann trial getting a lot of media attention, the suffix "of the european Jews" vanished more and more, and by the mid-60s the term Holocaust, at least in the USA, was understood as the term only describing this specific genocide.

In non-english speaking countries in western Europe, the term needed much more time. In fact, most of the adaptations of the term can be traced back to the TV mini-series Holocaust which confronted especially german people with the word and the crime itself. This movie got so much attention that two right-wing extremists even blasted TV towers to prevent Germans from watching the movie.

So, to sum things up: The word is old, and it has been used to describe what we know as the Holocaust even when it happened. It took some decades after 1945 to become the term only having the one meaning.

Unfortunately, I only know of one really good text for further reading (and I only recommend things I've read), and it's in German: "Von der Endlösung der Judenfrage zum Holocaust: Über den sprachlichen Umgang mit der deutschen Vergangenheit" by Gabriele von Glasenapp.

A side note: in Hebrew, it's not called the Holocaust, it's called Shoah--the catastrophe. In Yiddish it's Churban, and that was being used at least by 1945.