Halliday & Matthiessen (2014: 3):
When people speak or write, they produce text; and text is what listeners and readers engage with and interpret. The term ‘text’ refers to any instance of language, in any medium, that makes sense to someone who knows the language; we can characterise text as language functioning in context (cf. Halliday & Hasan, 1976: Ch. 1; Halliday, 2010). Language is, in the first instance, a resource for making meaning; so text is a process of making meaning in context.