Vygotsky's Zone of Proximal Development (ZPD): the space between what a learner can do independently and what they can do with expert assistance.
Instruction should target the ZPD — too easy produces no growth, too difficult produces frustration and learned helplessness. The ZPD is also an argument for social learning: a peer who is slightly more advanced can pull you through the ZPD in ways that a much more advanced teacher can't.
The concept is rich but hard to operationalize: the ZPD varies by domain, changes quickly, and can't be measured by standard assessments (which measure independent performance, not assisted performance).