Halliday (2007 [1991]: 271):
Originally, the context meant the accompanying text, the wording that came before and after whatever was under attention. In the nineteenth century it was extended to things other than language, both concrete and abstract: the context of the building, the moral context of the day; but if you were talking about language, then it still referred to the surrounding words, and it was only in modern linguistics that it came to refer to the non-verbal environment in which language was used. When that had happened, it was Catford, I think, who suggested that we now needed another term to refer explicitly to the verbal environment; and he proposed the term “co-text”.For the difference between material setting, context and co-text, click here.
For material setting vs context see here.
