Watershed (noun, adjective)
watershed [waw-ter-shed, wot-er-]

noun
1. Chiefly British. the ridge or crest line dividing two drainage areas; water parting; divide.
2. the region or area drained by a river, stream, etc.; drainage area.
3. Architecture. wash (def 44).
4. an important point of division or transition between two phases, conditions, etc.: The treaty to ban war in space may prove to be one of history's great watersheds.

adjective
5. constituting a watershed: a watershed area; a watershed case.

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Origin: First recorded in 1795-1805; water + shed

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