Truth be told I’ve only ever seen this use of the word synthesist used in science fiction. By a few authors, and it’s about half and half used either as a way of translating technical ideas to the reader, and/or as a way to ensure the narrator’s survival as he usually isn’t a specialist in any field, so he doesn’t stick his nose too deep in the muck. Then he can use his shallow knowledge of many fields to synthesize an escape plan.
In some cases a synthesist is an observer who mediates between scientists and the public, in other cases he is a jack of all trades for the crew.
