In the context of Maxwell's equations, it is vector field that is one of the inputs of the equation.
Section "Maxwell's equations with pointlike particles" asks if the theory would work for pointlike particles in order to predict the evolution of this field as part of the equations themselves rather than as an external element.
Measured in amperes in the International System of Units.
This can be used to detect the ionization of air by radiation , see e.g. youtu.be/CZ7DoLLwW04?t=76 from Video "Ions produced by radiation carry a current by Institute of Physics".
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An electric field is a region around a charged particle or object within which other charged particles experience a force. It is a vector field that represents the force per unit charge that a positive test charge would experience at any point in space.