Halliday & Matthiessen (2004: 587):
A text is thus a unit of meaning — more accurately, a unit in the flow of meaning that is always taking place at the instance pole of the cline of instantiation.
Instantiation Perspective: Text As Instance Of System Potential
Halliday & Matthiessen (2004: 23):
A text is the product of ongoing selection in a very large network of systems — a system network.
Halliday & Matthiessen (2004: 33):
In the form that we typically receive it, as spoken and written discourse, a text is the product of two processes combined: instantiation and realisation. The defining criterion is instantiation: text as instance. But realisation comes in because what becomes accessible to us is the text as realised in sound or writing. We cannot directly access instances of language at higher strata — as selections in meaning, or even in wording.
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.
Stratification Perspective: Text As Semantic Stratum Unit
Halliday & Matthiessen (2004: 588, 588n):
The upper bound of the semantic stratum is the text: this is the most extensive unit of meaning. The upper bound of the lexicogrammatical system is the clause: this is the most extensive unit of wording. … By saying that they are the upper bounds, we are not ruling out complexes — text complexes and clause complexes … . But complexes are not higher–ranking units but rather expansions of units of a given rank.
Halliday & Matthiessen (1999: 122-3):
Texts and sequences are of the same order of abstraction; both are semantic phenomena. A text is a piece of language that is functional in context. It draws on the ideational meaning base but it involves the full metafunctional spectrum; i.e. there are interpersonal and textual contributions as well. Since text draws on the ideational meaning base, sequences are one principle for organising text.