define magnetic field

A magnetic field is a vector field that describes the magnetic influence of electric charges in relative motion and magnetized materials. The magnetic field at any given point is specified by both a direction and a magnitude (or strength), so it is a vector field. The term is used for two distinct but closely related fields denoted by the symbols B and H, where H is measured in units of amperes per meter (symbol: A/m) in the SI. B is measured in teslas (symbol: T) and newtons per meter per ampere (symbol: N·m−1·A−1) in the SI. B is most commonly defined in terms of the Lorentz force it exerts on moving electric charges.