Tap water is drinking water that comes from an indoor tap or spigot. It is part of a larger indoor plumbing system that requires a complex supportive infrastructure, including a stable water source, piping, and water filtration to keep the water safe. Indoor plumbing is very common in developed nations, but its distribution is more varied in the developing world, where the infrastructure to support access to drinking water is not always present.
Indoor plumbing really began to flourish in the 1800s, although many societies had experimented with various versions of indoor plumbing prior to this period. The primary advantage to indoor plumbing, of course, is that it makes water readily accessible for cooking, washing, cleaning, and drinking, but it is also often designed with water safety in mind. Water safety can be controlled at a source like a reservoir or well, at a water treatment plant along the way, or at the house, depending on the type of plumbing system involved.
Tap water can be routed through a water heater to warm it before it reaches the tap, or it may be run in a simple cold line. There are a number of ways to force water to the tap, including using a water pump to push water through the plumbing, or using a pressurized water system, such as the system used by many cities to maintain water pressure, ensuring that as soon as the tap is opened, water will flow out.
The safety of this water can be compromised by any number of things. In some regions, water purification systems are insufficient, allowing potential pathogens and other forms of contamination into the supply. In other regions, water may become contaminated somewhere along the way to the tap, as might be the case when a sewer pipe bursts, allowing dirt and other contaminants into lines used to supply water to homes, or when home pipes become corroded, adding metals and other materials to the water.
Many people in the developed world take access to tap water for granted, but in other regions, it may not be so accessible. In some areas, water is only available for several hours a day, forcing people to plan their schedules around the times when the taps will actually yield water, and in other regions, indoor plumbing is still a long way off, as is access to clean water in any form.