A geomagnetic storm is a short-term disruption of the magnetic field
surrounding Earth, i.e., a temporary change from its usual
state, typically returning to normal in a few days.
Such storms are invariably caused by the Sun, basically a
solar storm (an episode of solar activity)
that specifically affects the Earth's magnetic field,
such as a coronal mass ejection (CME) that happens to face Earth.
Such storms can affect radio communication, power, satellites,
etc., can damage electronics, and often include auroras.