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It is straight above where an earthquake originates. It is contrasted with the hypocenter, the actual location of the quake inside the earth. Shadows occur on the opposite side of the Earth from the earthquake epicentre because the outer core reflects S-waves, and bends P-waves.
It is straight below the point of an air or surface detonation of a nuclear weapon and it is strictly above the point of the underground detonation.
The terminology is also applicable to other catastrophic events, such as an impact or explosion of a meteorite or in relation to astronomic bodies other than the Earth.