Chiefly British . the ridge or crest line dividing two drainage areas; water parting; divide.

the region or area drained by a river, stream, etc.; drainage area.

Architecture . wash ( def. 44 ) .

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.

constituting a watershed

a watershed area; a watershed case.