Systemic functional linguistics (SFL) is an approach to linguistics, among functional linguistics, that considers language as a social semiotic system.
It was devised by Michael Halliday, who took the notion of system from J. R. Firth, his teacher (Halliday, 1961). Firth proposed that systems refer to possibilities subordinated to structure; Halliday "liberated" choice from structure and made it the central organising dimension of SFL. In more technical terms, while many approaches to linguistic description place structure and the syntagmatic axis foremost, SFL adopts the paradigmatic axis as its point of departure. Systemic foregrounds Saussure's "paradigmatic axis" in understanding how language works. For Halliday, a central theoretical principle is then that any act of communication involves choices. Language is above all a system; SFL maps the choices available in any language variety using its representation tool of a "system network".