Magnetic induction (Electromagnetic induction) is the creation of a voltage across an expanse of electrically conductive material caused by a change in a surrounding magnetic field. It is a consequence of Maxwell's equations (specifically, Ampere's law) and is the basic mechanism of electrical generators and a key part of electrical transformers.
Magnetic induction occurs in astronomical phenomena when a magnetic field is changing (e.g., a rotating object with a magnetic field misaligned with the axis of rotation) and can also occur when a conductive material is moving through a magnetic field. It is also a key part of magnetohydrodynamics.
Magnetic induction is triggered by a conductive object passing through the gradations of a magnetic field, and is thus a primary means to measure the magnetic field around solar system planets and moons by space probes passing near them. Data so-collected can help in working out the overall magnetic field (e.g., by a working dynamo) as well as specific details. For example, it has been used as a means to detect subsurface water in solar system moons: if the host planet has a misaligned magnetic field and the moon's subsurface water is conductive (e.g., salty), a voltage is induced in the water, which, in turn, modifies the surrounding magnetic field, potentially beyond the surface of the moon. This gives one means an external probe can provide evidence of internal characteristics of a moon. Europa is an example.
The term magnetic induction is also sometimes used as a synonym for the related measure, magnetic flux density.