Mediation is the central claim of Vygotsky's psychology: humans do not act on the world, or on their own minds, directly, but always through something. Between a person and the object of their activity sits a mediating means, and that means reshapes the activity. Vygotsky's signature example is humble. A person who ties a knot in a handkerchief to remember an errand has reorganised their own memory from the outside, using an external sign to control an internal function. The natural, unmediated memory is gone; in its place is a memory steered by a cultural tool.
Vygotsky distinguished two kinds of mediator. Material (technical) tools are turned outward and transform objects in the world. Psychological tools, or signs, are turned inward and transform the mind, and the master sign is language. Numbers, diagrams, mnemonics, maps, and above all words let a person regulate their own attention, memory, and reasoning rather than being at the mercy of the immediate stimulus. As higher mental functions develop, they are progressively brought under the control of these symbolic means.
Work in sociocultural SLA (Lantolf and colleagues) treats three strands of mediation as the engine of L2 development:
The pedagogical point is that the support is not external scaffolding around "real" learning; the mediated activity is the learning, the site where capability is first assembled socially before being internalised. Assessment built on this premise, dynamic assessment, deliberately measures how a learner responds to mediation rather than what they can do unaided.