Michael Alexander Kirkwood Halliday (often M. A. K. Halliday; 13 April 1925 – 15 April 2018) was a British linguist who developed the internationally influential systemic functional linguistics (SFL) model of language. His grammatical descriptions go by the name of systemic functional grammar. Halliday described language as a semiotic system, "not in the sense of a system of signs, but a systemic resource for meaning". For Halliday, language was a "meaning potential"; by extension, he defined linguistics as the study of "how people exchange meanings by 'languaging'". Halliday described himself as a generalist, meaning that he tried "to look at language from every possible vantage point", and has described his work as "wander[ing] the highways and byways of language". But he said that "to the extent that I favoured any one angle, it was the social: language as the creature and creator of human society".

Halliday's grammar differs markedly from traditional accounts that emphasise classification of individual words (e.g. noun, verb, pronoun, preposition) in formal, written sentences in a restricted number of "valued" varieties of English. Halliday's model conceives grammar explicitly as how meanings are coded into wordings, in both spoken and written modes in all varieties and registers of a language. Three strands of grammar operate simultaneously. They concern: (i) the interpersonal exchange between speaker and listener, and writer and reader; (ii) representation of our outer and inner worlds; and (iii) the wording of these meanings in cohesive spoken and written texts, from within the clause up to whole texts. Notably, the grammar embraces intonation in spoken language. Halliday's seminal Introduction to Functional Grammar (first edition, 1985) spawned a new research discipline and related pedagogical approaches. By far the most progress has been made on English, but the international growth of communities of SFL scholars has led to the adaptation of Halliday's advances to some other languages.