Halliday & Matthiessen (1999: 375):
Context is the 'semiotic environment' of language (and other sociosemiotic systems such as image systems [maps, diagrams, etc.]); its systems specify what demands may be placed on language and what role it may play in responding to those demands. There are three sets of contextual systems — field, tenor and mode; … Recurrent combinations of (ranges of) field, tenor and mode values define regions within the overall system of the context of culture.