An open system is one whose border is permeable to energy (or mass) as distinct from a closed system in which the border is not permeable. Note that the definition of a "system" is arbitrary; a system is simply a collection of components (that have elements related in some way) that is under study.
Open systems have a number of consequences. In a closed system, there exists limited energy. In open systems, there exists supplies of energy that cannot be depleted. In a technical sense, open system means a circuit that includes the environment. This type of system is sometimes referred to as "radiant energy". Some open system have forms of the energy that can be receive from space via vacuum energy (sometimes referred to as "zero-point").
Examining processes with knowledge of open system mechanics can lead to interesting insight. An example of an open system is the space tether, which uses the ground and the ionosphere as parts of a circuit to extract energy from the Earth's magnetic field by electromagnetic induction. Open systems can occur in suspended circuits, in which the conductor has no ground and uses the environment to complete a circuit. Open systems can also occur in grounded circuit, in which the conductor has a ground and uses the environment to complete a circuit.
During discussions of perpetual motion, the topic of open system usually encourages serious inquiries.