The voltage, measured as the root mean squared value (RMS), between two slots or holes in an electrical outlet of the sort typically mounted in a wall. Standard wall voltages in most of the world are either 110 or 220. Typical residential installations in the United States protect the wall outlets with 15 Amp or 20 Amp circuit breakers.
Wall voltage will vary a bit depending on the load that the power grid must handle. Higher loads will reduce the available voltage, and the North American Electric Reliability Council (NERC) regulates how much the voltage is allowed to vary. Typically, loads increase during the workday while most business are open, and usually increase significantly during warm weather when air conditioners are in operation.
The electric company regulates the voltage across the power grid in two ways. Large fluctuations in load are handled by increasing the power output of the generators. Smaller fluctuations can be controlled by switching on capacitor banks around the power distribution centers. These capacitors raise the voltage in a small area when they are switched on.
A brownout occurs when the system is overloaded, and the wall voltage drops below acceptable levels.