Effluent is an outflowing of water from a natural body of water, or from a man-made structure.

Effluent in the man-made sense is generally considered to be pollution, such as the outflow from a sewage treatment facility or the wastewater discharge from industrial facilities. An effluent sump pump, for instance, pumps waste from toilets installed below a main sewage line.

In the context of a nuclear power plant or nuclear processes such as creating plutonium at the Hanford Site, effluent refers to the cooling water that is discharged from a nuclear reactor that may or may not be radioactive. This water is generally not physically contaminated, but is usually warmer than the body of water it discharges into.

In Sugar Beet processing, effluent is often settled in water tanks which allow the mud-contaminated water to settle. The mud sinks to the bottom, leaving the top section of water clear, free to be pumped back into the river or be reused in the process again.

The Mississippi River's effluent of fresh water is so massive (7,000 to 20,000 m³/sec, or 200,000 to 700,000 ft³/sec) that a plume of fresh water is detectable from outer space, even as it rounds Florida and up to the coast of Georgia.