“Tap water” is any water that comes out of a tap, a valve that controls the release of a liquid or gas. A water tap can be a faucet, spigot, or hydrant and expels water used not just for drinking, but also for washing, cooking, and toilet-flushing. It is distinct from other types of fresh water, such as water coming from a cistern, town pump, or directly from rivers, lakes, and other bodies of water. While tap water is usually potable, it is not unusual to discover water quality issues, and many households utilize some form of water filtration system.
Generally speaking, in the United States, tap water refers to potable (as in, safe to drink by humans) water that comes from some sort of municipal water supply and is delivered to a home, school, or business (etc.) with no provider between the two end points. It comes from a faucet (though it may be filtered before or after the faucet) as opposed to being bottled.
Tap water treatment and quality is monitored by government agencies. The water is processed to remove contaminants and adjust pH levels. However, tap water can still become contaminated. The water crisis in Flint, Michigan, is a recent, and extreme, example of such contamination. Causes of contamination are biological (such as bacteria, viruses, or disease-causing toxins) or chemical (such as drugs, nitrogen, or pesticides). In the United States there is a general assumption that tap water is safe to drink, but that isn’t always the case… and it certainly is not the case in all parts of the world, especially if you are a traveler.
The EPA requires chlorine to be added to tap water as a disinfecting agent to kill pathogens commonly present in water mains and storage tanks. Fluoride may be added as well, for its tooth decay prevention properties, although in recent years this practice has come under fire, and there are several water filters on the market at various price points that remove both chlorine and fluoride.
While tap water is generally considered to be safe, and even high-quality, it’s always a good idea to monitor the water in one’s zip code for quality and any advisories in place.