watershed, n. [Cf. G. wasserscheide; wasser water + scheide a place where two things separate, fr. scheiden to separate.].
The whole region or extent of country which contributes to the supply of a river or lake. [1913 Webster]
The line of division between two adjacent rivers or lakes with respect to the flow of water by natural channels into them; the natural boundary of a basin; -- called also divide and water parting. [1913 Webster]
a point in time marking an important transition between two situations, or phases of an activity; a turning point. [PJC]