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A coronal mass ejection (CME) is a large mass of plasma with strong magnetic field lines ejected from the Sun, i.e., a massive burst of solar wind. They can be thought of as the largest activity of the Sun that occurs regularly within our observing timescale, their frequency ranging from about one per week at solar minimum to two-to-three times a day during solar maximum. They constitute the solar storms destructive of electronics and electrical infrastructure, but Earth is a relatively small target, completely missed by the vast majority. They vary in power and destructiveness, with an extremely destructive one striking Earth perhaps every 25 years, and other destructive ones perhaps every 3 years.
The same term is also used for the same phenomenon on other stars.