According to Vygotsky what is the zone of proximal development (ZPD)?

ZPD refers to the range of tasks that an individual cannot yet do alone but can accomplish when ASSISTED BY OTHERS (Berger, 2012; Gredler, 2012). Vygotsky (1978, p.106) himself described it as "the distance between the actual development 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".