By definition, a holocaust is "destruction or slaughter on a mass scale" (definition from google)
Is there any reason in particular that we only generally use the word to describe either nuclear destruction or the killing of Jews during WWII? Stalin killed many more of his own countrymen. What about Mao?
This is inspired by a post on the front page today that uses the word holocaust to describe what's going on in North Korea. Wouldn't it, then, be appropriate to use the word to describe all of the other atrocities that resulted in dozens of millions of deaths from dictators like Stalin and Mao? Or is it inappropriate to compare what's going on in North Korea to the Final Solution?
I found an old post on /r/askhistorians asking a similar question, and the top comment was that the WWII holocaust was a systematic, intentional destruction of a race of people, and that's what sets it apart from the likes of Stalin et al. How is modern day North Korea different from USSR Stalin?
The word holocaust actually comes from a Greek word which means 'wholly burnt'. This refers to an ancient animal sacrifice in which the animal is completely burnt (an example being Noah sacrificing lambs in the Hebrew Bible).
Whilst the word is now synonymous with the systematic killings by the Nazis, it isn't the only term used. Jews refer to it as the 'calamity' (Shoah) to differentiate it with the sacrificial holocaust of the Bible.
As seen in the source below (New York Times, May 23, 1943 - Page 12) the writer refers to it as the 'Nazi holocaust'. This is one of, if not the, first recorded instance where the Holocaust bears it's common usage.
http://www.freerepublic.com/focus/chat/3022901/posts
Fast forward time to the late 70s, where the critically acclaimed series 'Holocaust' was on the TV and here we see the word truly become synonymous with the Nazis' Final Solution as the word is named by Spiegel the 'word of the year'. Quote is in German but translated through google - http://translate.google.co.uk/translate?hl=en&sl=de&u=http://einestages.spiegel.de/static/topicalbumbackground/23795/ein_jahr_ein_un_wort.html&prev=/search%3Fq%3DEin%2BJahr,%2Bein%2B%28Un-%29Wort!%2B1977%26safe%3Doff%26client%3Dfirefox-a%26hs%3D3xI%26rls%3Dorg.mozilla:en-US:official%26channel%3Drcs%26biw%3D1600%26bih%3D797
Hope this answers your question well :)
Is there any reason in particular that we only generally use the word to describe either nuclear destruction or the killing of Jews during WWII? Stalin killed many more of his own countrymen. What about Mao?
I think you're confusing the meaning of a holocaust with the Holocaust. Words for general things can also refer to specific things in the right context. The best example is that a moon (lowercase) is a natural satellite of a planetary body, but the Moon (uppercase) is earth's natural satellite. The other comment already here explains a bit about how the Holocaust came to be the proper noun for one particular mass killing.
In a theoretical way there's nothing wrong with using a holocaust as a general term for mass killing. But since it goes against most common modern usage (edit: which uses term only for killing during WW2 or nuclear war, as you said), there isn't any reason to either.
The definition of "destruction or slaughter on a mass scale" is a relatively new one, and became prominent because it was used to refer to the genocide of Jews in Europe. As someone else mentioned, "holocaust" originally referred to complete destruction by fire of a sacrifice. So the reason that the term isn't generally used to refer to Stalin or Mao's killings, is that once this relatively obscure word came to be used to refer to the Shoah, it became closely identified with that event and seemed out of place in other contexts. Plus the Shoah and nuclear destruction both involve fire, which was a key component of the definition of the term originally. Whereas "genocide" seems more generic, to be used for more situations. You can see in this Google Ngram viewer how the word grew in popularity after WWII: https://books.google.com/ngrams/graph?content=holocaust&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cholocaust%3B%2Cc0
As a side note: I believe the reason why people have started to use "holocaust" to refer to the situation in North Korea is that the North Korean detention camps are very very similar to Nazi concentration camps, so the parallels are obvious. The deaths that occurred under Stalin and Mao's watches differed in many respects from the Holocaust, even though they involved massive numbers of dead people. So an evocative word like "Holocaust" can end up being misleading if used in those contexts.
Part of the issue is that the Holocaust has too much historical connotation to a given event (the Holocaust by the Nazis in Germany)
I think the word genocide has a better use here, not only because it covers the literal discussions of mass murder and extermination, but it also includes cultural genocide as well.
In this way, the Canadian government's treatment of native people is seen by many historians, myself included, as cultural, and in some cases, biological genocide.