Inner speech is the silent, self-directed language a person uses to think with, and in Vygotsky's account it is where the social origins of mind become most visible. His claim, developed in the final chapter of Thinking and Speech, is that inner speech is not thought that happens to be wordless, nor speech minus the sound. It is a distinct internal function with its own grammar, formed by a developmental journey from outside to inside.
That journey runs through three phases. Speech begins as external (social) speech, used to communicate with others. It then passes through egocentric (private) speech, the audible running commentary young children produce while working through a task, talking to no one in particular. Vygotsky argued, against Piaget, that this self-talk is not a sign of immature social isolation that simply withers away; it is social speech turning inward to do a new job, the regulation of one's own activity, and rather than disappearing it goes underground to become silent inner speech.
Inner speech has a recognisable shape. It is radically abbreviated and predicative: because the speaker already knows the subject, only the new, predicate-like part survives, so whole trains of thought collapse into a word or two. Meaning runs ahead of dictionary definition, with the personal sense of a word swamping its shared meaning. This compression is exactly what makes inner speech an efficient instrument for thinking.
Sociocultural SLA takes private and inner speech seriously as mediation of the self. A learner muttering through a grammar exercise, rehearsing a phrase under their breath, or talking themselves through a writing plan is not off task; they are using the L2 (or the L1) to regulate their own performance, and that self-regulatory talk is one of the routes by which social support gets internalised. The phenomenon connects to languaging, Swain's idea that producing language is itself a means of making meaning and not merely a report on meaning already made.