watershed

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A ridge of high land dividing two areas that are drained by different river systems. Also called water parting.
2. n. The region draining into a river, river system, or other body of water.
3. n. A critical point that marks a division or a change of course; a turning point: "a watershed in modern American history, a time that ... forever changed American social attitudes” ( Robert Reinhold).

Century Dictionary and Cyclopedia

1. n. The area drained by a river and all its tributaries.
2. n. The edge of a river-basin (see river); the line separating the waters flowing into two different rivers or river-basins. Thus, the crest of the Sierra Nevada of California forms the watershed between the rivers flowing into the Pacific and those which lose themselves in the Great Basin. Sometimes called the water-parting, and in the United States more frequently and popularly the divide. Thus, the “Continental Divide” is the line which marks the separation of the waters flowing into the Pacific from those finding their way to the Gulf of Mexico.

Wiktionary

1. n. hydrology The topographical boundary dividing two adjacent catchment basins, such as a ridge or a crest.
2. n. US A region of land within which water flows down into a specified body, such as a river, lake, sea, or ocean; a drainage basin.
3. n. A critical point marking a change in course or development.
4. n. Canada, UK The time after which material of more adult nature (violence, swear words, sex) may be broadcast on television or radio, either one laid down or one contrived (e.g. when children are not watching...)

GNU Webster's 1913

1. n. The whole region or extent of country which contributes to the supply of a river or lake.
2. n. 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.
3. n. a point in time marking an important transition between two situations, or phases of an activity; a turning point.

WordNet 3.0

1. n. the entire geographical area drained by a river and its tributaries; an area characterized by all runoff being conveyed to the same outlet
2. n. a ridge of land that separates two adjacent river systems
3. n. an event marking a unique or important historical change of course or one on which important developments depend