Spatial disorientation, spatial unawareness is the inability of a person to correctly determine his/her body position in space. This phenomenon refers especially to aircraft pilots and underwater divers, but can be also induced in normal conditions — chemically or physically (e.g. by blindfolding). In aviation, the term means the inability to correctly interpret aircraft attitude, altitude or airspeed, in relation to the Earth or point of reference, especially after a reference point (e.g., the horizon) has been lost. Spatial disorientation is a condition in which an aircraft pilot's perception of direction does not agree with reality. While it can be brought on by disturbances or disease within the vestibular system, it is more typically a temporary condition resulting from flight into poor weather conditions with low or no visibility. Under these conditions the pilot may be deprived of an external visual horizon, which is critical to maintaining a correct sense of up and down while flying.