Internalisation is Vygotsky's account of how something a learner first does with other people becomes something they can do alone. It is the mechanism that links the social and the mental, and it is stated most famously as the general genetic law of cultural development: "Any function in the child's cultural development appears twice, or on two planes. First it appears on the social plane, and then on the psychological plane. First it appears between people as an interpsychological category, and then within the child as an intrapsychological category" (Vygotsky, 1978, p. 57).
The move from the interpsychological to the intrapsychological is not a copy. Vygotsky insisted that internalisation transforms what is internalised: the function is reconstructed on a new plane, not photographed and filed. A learner who is talked through a task by a teacher does not store a recording of the teacher's words; they rebuild the regulating talk as their own inner speech, reorganised and abbreviated for private use. Internalisation is therefore a process spread over time, not an event, and it runs through mediated activity in the ZPD.
For sociocultural SLA, internalisation is what acquisition is: the learner gradually takes over the L2 forms, strategies, and self-regulatory talk that were first supplied socially, through interaction and support, until they can deploy them independently. This is why collaborative dialogue, private speech, and graduated help are treated as developmentally central rather than as mere practice around the edges of learning.
Anna Stetsenko argues the mainstream reading flattens the idea into passive absorption, a lone individual soaking up the surrounding culture, which she takes to be a reversal of Vygotsky's intent. On her transformative account, internalisation is better seen as turning toward oneself as a partner in shared, world-changing activity, keeping the collective and agentive character the original Marxist framing gave it (see The Americanisation of Vygotsky).