A boiler retort is the heated pressure vessel in which steam pressure is produced.
A distillation retort is the vessel that holds the material from which the desired product is distilled, even if that retort happens to be steam heated from a boiler.
My experience is with vacuum furnaces, both debinding and reduction. The vacuum furnace retort is an optically tight but not air tight chamber within the main vacuum chamber and within the bounds of the heating elements. The retort contains the material being processed. As the material is processed the retort helps assure that out-gassed compounds find their way into the evacuation manifold and are removed through the vacuum pump, instead of wandering over to the chamber walls to condense.
As you can see, the definitions of "retort" vary widely, but the one common thing is that it is the chamber that holds the material being processed.