Every process has associated with it one participant that is the key figure in that process; this is the one through which the process is actualized, and without which there would be no process at all. Let us call this element the MEDIUM, since it is the entity through the medium of which the process comes into existence.
- Halliday (1985), a pag.146

[...] the Medium is obligatory in all processes [...] The Medium is also the only element that is never introduced into the clause by means of a preposition [...] it is treated as something that always participates directly in the process.
- Halliday (1985), a pag.146-147

[...] The Medium is the nodal participant throughout: not the doer, or the causer, but the one that is critically involved, according to the particular nature of the process.
- Halliday (1985), a pag.147