When and where was the term 'Holocaust' first used to describe what occurred in the death camps?

As far as can be established, the word 'Holocaust' (actually 'a holocaust', not 'the Holocaust') was first used in the (London) News Chronicle in November 1942 to describe the extermination of the Jews.

The word 'genocide' was first used in print in 1944. However, the term Holocaust was not widely used for the Nazi genocide of the Jews till the late 1970s following the broadcast of the TV miniseries with that title. (It is sometimes said the use of the term for the Nazi genocide of the Jews was popularized by Elie Wiesel from the late 1950s on).

Note the term Holocaust does not refer only to murder in death camps, but also to mass open-air shootings (of which there were many in Eastern Europe) and to deliberate starvation in ghettos.