A filtration apparatus is for example a separation device.
There is one line, the equator, that separates the northern from the southern hemisphere.
divide
divides
The English Channel.
A centrifuge.
A watershed divide is a ridge of land that separates one watershed from another by directing the flow of surface water in different directions.
One way valve
The line that separates one place from another is called a boundary. Boundaries can be physical, such as rivers or mountains, or they can be imaginary lines agreed upon by nations and jurisdictions.
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The peritoneum (and the mesentary).
Bass Strait separates mainland Australia from the island state of Tasmania.