How is a sewer defined?

Answer 1

A sewer is a man-made, usually underground channel for conveying liquid and solid waste from homes and businesses to a water purification plant.

A channel that carries semiliquid sewage from one place to another. In most modern countries sewers are enclosed and typically buried and drain into a sewage treatment plant, but in some places they are just open trenches on the surface that drain directly without any treatment into rivers or lakes that are often also used for drinking water.

Another type of sewer, the storm sewer, is simply a channel that carries storm water runoff from the streets to a body of water. These sewers (even in modern countries) often do not go through a treatment plant and thus should NEVER be used for disposal of any kind of waste (especially not waste motor oil)!