A very clear definition

Most of the psychological investigations concerned with school learning measured the level of mental development of the child by making him solve certain standardised problems. The problems he was able to solve by himself were supposed to indicate the level of his mental development at the particular time ... We tried a different approach. Having found that the mental age of two children was, let us say eight, we gave each of them harder problems than he could manage on his own and provided slight assistance ... We discovered that one child could, in cooperation, solve problems designed for twelve year olds, while the other could not go beyond problems intended for nine year olds. The discrepancy between a child's mental age [indicated by the static test] and the level he reaches in solving problems with assistance is the zone of his proximal development. (p. 103)

But one should also consult Lave and Wenger's discussion of the more implicit implications of this observation (1991).