Zone of proximal Development (ZPD) refers to Lev Semionovich Vygotsky's concept.
It is the difference between the developmental level a child has reached and the level she is potentially capable of reaching with the guidance or collaboration of a more skilled adult or peer. In theory, then, so long as a person has access to a more capable peer, any problem can be solved.
Vygotsky's concept of the zone of proximal development (ZPD) posits that human potential is theoretically limitless; but the practical limits of human potential depend upon quality social interactions and residential environment. This zone of proximal development is "the distance between the actual developmental level as determined by independent problem solving and the level of potential development as determined through problem-solving under adult guidance or in collaboration with more capable peers."(Lev Vygotsky, Mind in Society, edited by M. Cole, V. John-Steiner, S. Scribner, and E. Souberman (Cambridge, MA: Harvard University Press, 1978), 85-86).