Holocaustis derived from the Greek word "holokauston" which originally meant a sacrifice totally burned by fire. In the 1950's the term came to be applied to the destruction of the Jews of Europe by the Nazi German state.
"Holocaust" is also used to describe the annihilation of other groups during World War II.
The Hebrew word "Shoah" meaning catastrophe or destruction also denotes the attempt to destroy European Jewry during WWII. "Shoah" first appeared in this context in a booklet concerned with aid for the Jews of Poland published in Jerusalem in 1940.
Source: Encyclopedia of the Holocaust.