Halliday & Matthiessen (1999: 536):
Our basic approach to this is embodied in the term “metaphor”, as used in the context of metaphor in the grammar. We used the expression “grammatical metaphor” to refer to a complex set of interrelated effects whereby, in English and many other languages, there have evolved what seem to be alternative representations of processes and properties. In terms of word classes, meanings prototypically construed as verbs or adjectives come to be construed as nouns instead. But, as we saw, this is simply the superficial manifestation of a wider and deeper phenomenon affecting the entire construal of experiential meanings in the grammar.