1. In communications terms, a blackout is a fade-out of radio communications due to environmental factors such as ionospheric disturbances or a plasma sheath surrounding a reentry vehicle.
2. In physiology, a blackout is a condition in which vision is temporarily obscured by blackness, accompanied by a dullness of certain other senses, brought on by decreased blood pressure in the head and a consequent lack of oxygen. It may occur, for example, as a result of being exposed to high g-forces in an airplane or spacecraft (see anti-g suit).