In electrical engineering, ground or earth is the reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the earth.
For measurement purposes, the Earth serves as a (reasonably) constant potential reference against which other potentials can be measured. An electrical ground system should have an appropriate current-carrying capability to serve as an adequate zero-voltage reference level. In electronic circuit theory, a "ground" is usually idealized as an infinite source or sink for charge, which can absorb an unlimited amount of current without changing its potential. Where a real ground connection has a significant resistance, the approximation of zero potential is no longer valid. Stray voltages or earth potential rise effects will occur, which may create noise in signals or if large enough will produce an electric shock hazard.
The Earth's ground is used as an electrical reference point for circuits. Grounded electronics see the Earth as zero volts. The power company connects one port in each grounded outlet straight to the Earth outside. That connection can sometimes be seen outside as a metal stake driven into the dirt.
Electronic devices that don't plug into a wall socket have a different type of ground usually called common ground. A circuit that runs on batteries will often use the battery's negative terminal as common ground - a reference point of zero volts compared to the battery's positive terminal.