Watershed has more than one meaning:

• In physical geography, watershed has two distinct meanings:

• A watershed can be a drainage basin, especially in North American usage. This is the region of land whose water drains into a specified body of water.

• A watershed can be a drainage divide, especially in Britain and other Commonwealth countries. This is the ridge of land that separates two adjacent drainage basins.
  • A watershed (television), in Canada, Ireland, South Africa and the United Kingdom, is the time of day before which programme content of a specified or implied kind may not be screened and after which it is permissible. It is a figurative use of the word deriving from its original meaning as a boundary between drainage basins.
• A watershed (or weathershed) is the upper surface of a ribbed high voltage electrical insulator that deflects and drains ice and water away from the bottom portions and central body of the insulator. Watersheds break up the flow of water, thereby preventing a continuous channel of water from bridging, and possibly shorting, the insulator.
• A watershed or watershed event is any moment or event separating two distinct periods of time.
• Watershed is a South African band featuring lead singer Craig W. Hinds, most famous for their signature tune 'Indigo Girl', earning them much international exposure, most successfully in Germany.
• Watershed is an American band based in Columbus, Ohio
• Water Shed 5tet, a band led by saxophonist Ben Opie, based in Pittsburgh, originally known as Watershed.
• The Watershed Media Centre is a media and arts centre in Bristol.
• Watershed is a maker of submersible waterproof duffels, backpacks, and other soft cases.

Etymology of watershed

The first recorded instance of the term watershed was in 1803. The definition ascribed is as a dividing line between two river basins (water divide):

"Strathcluony..is a very high inland tract, being the water-shed of the country between the two seas."Oxford English Dictionary

This looks like an invented analogue of the much longer-established German word, Wasserscheide: water divide.

The term soon developed an alternate and closely related meaning as "the slope down which the water flows from a water-parting,"Oxford English Dictionary, which corresponds to the area of surface runoff, and excludes channel flow. This meaning was ascribed to the following quote:

"To the south~west of Kington the lower beds of the Old Red Sandstone..have been the sub-aqueous water-shed, down which the coarse detritus has been swept."Sir Roderick Impey Murchison, The Silurian system, founded on geological researches, (1839)

These closely-related meanings led Thomas Henry Huxley, in 1877, to remark the following:

"To avoid all ambiguity it is perhaps best to set aside the original meaning of ‘watershed’, and employ the term to denote the slope along which the water flows, while the expression ‘water-parting’ is employed for the summit of this slope". T.H. Huxley, Physiography: An Introduction to the Study of Nature, (1877)

By 1874 the definition had expanded further, to become distinctly synonymous with the drainage basin as a whole, and not just the upper slopes.

"The Missouri Region, in its broadest sense, as embracing the whole water~shed of that great river and its tributaries."Elliott Coues, Birds of the Northwest, (1874)

The figurative use of watershed as a moment separating two distinct periods, following the first and second definition, was adopted by 1878:

"Midnight! the outpost of advancing day!.. The watershed of Time, from which the streams of Yesterday and To-morrow take their way.Henry Wadsworth Longfellow, Kéramos, and other poems", (1878)

References