Effluent is defined as liquid waste that is untreated, partially treated, or completely treated. It is usually wastewater from sewers or industrial outfalls that flows directly into surface waters, either untreated or after being treated at a facility. The term "effluent" has slightly different meanings in certain contexts and may contain various contaminants. In general, effluent is considered to be water pollution, such as the outflow from a sewage treatment facility or an industrial wastewater discharge.

Effluent can be produced and discharged by any industrial or commercial premises, and it usually flows from the premises directly into the main sewer network. It cannot enter a river, reservoir, stream, or lake unless it is cleaned and treated first. The United States Environmental Protection Agency (EPA) defines effluent as "wastewater–treated or untreated–that flows out of a treatment plant, sewer, or industrial outfall. Generally refers to wastes discharged into surface waters".

In summary, effluent is liquid waste that is discharged from various sources, including industrial and commercial premises, and it can contain various contaminants. It is usually wastewater from sewers or industrial outfalls that flows directly into surface waters, either untreated or after being treated at a facility.