Halliday & Matthiessen (2004: 524):
Text is something that happens in the form of talking or writing, listening or reading. When we analyse it, we analyse the product of this process; and the term ‘text’ is usually taken as referring to the product — especially the product in its written form, since this is most clearly perceptible as an object … . The organisation of text is semantic rather than formal, and much looser than that of grammatical units. The organisation of text has typically been represented in some form of structural notation. But it is important to to be able to think of text dynamically, as an ongoing process of meaning.