water mixed with waste matter
"The treatment plant processes wastewater before releasing it back into the river."
Any water that has been used in some human domestic or industrial activity and, because of that, now contains waste products.
"The city's treatment plant processes millions of gallons of wastewater from homes and factories before releasing it back into the river."
In plain English: Wastewater is dirty water that has been used for things like washing, flushing toilets, and cooking before it gets treated so it can be safely released back into the environment or reused.
"The factory was fined for illegally dumping its wastewater into the local river."
Usage: Wastewater refers to any water contaminated by household or industrial use rather than clean drinking supply. It is commonly distinguished from rainwater runoff when discussing sewage treatment systems.
The word wastewater is a straightforward combination of the words waste and water. It was formed to describe any liquid that has been used or contaminated before being discarded.