wa⋅ter⋅shed
   /ˈwɔtərˌʃɛd, ˈwɒtər-/ Show Spelled Pronunciation [waw-ter-shed, wot-er-] Show IPA
–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.