The word holocaust has been used in English from as early as the 1300s in translations of the Bible, where it means burnt offering. It derives from ὁλόκαυτος the Greek translation of various Hebrew words referring to a burnt sacrifice. From the 1600s the word holocaust started to be used outside of the Bible and to expand from something that is wholly consumed by fire, to a massacre of people by fire, to a massacre in general. By the early 20th century the use of "holocaust" to refer to the killing of a great number of people was well established. Therefore, it is not surprising that it was used as early as 1942 when news of the killing of the Jews by the Germans reached the West. The word continued to be used for other kinds of massacres throughout the 40s and 50s, notably the dreaded "nuclear holocaust".

The changeover from holocaust as a general term for mass murder to the Holocaust, the killing of the Jews by the Germans was gradual. Two key factors in the change were the frequent use by Israel's Yad Vashem research institute of the term, and the Eichmann trial which served to bring the full scope and tragedy of the killings to the fore, really for the first time since the end of the war.

Sources: OED entry for Holocaust and The Secular Word "HOLOCAUST": Scholarly Sacralization, Twentieth Century Meanings by John Petrie.