Halliday & Matthiessen (2014: 336, 336n):
… the medium through which the process is actualised.*
* Halliday (1968: 185/2005: 117) originally suggested the term ‘affected’ for what is now called ‘medium’, although Fawcett and other linguists working with and developing descriptions within the ‘Cardiff Grammar continue to use the term ‘affected’ in their accounts. Within other linguistic frameworks, something like the role of Medium has been characterized by means of other labels. For example, in Starosta’s (1988: 128) Lexicase theory, ‘“Patient” corresponds to Halliday’s medium’.