Spatial disorientation is the inability to determine position or relative motion, commonly occurring during periods of challenging visibility, since vision is the dominant sense for orientation. The auditory system, vestibular system (within the inner ear), and proprioceptive system (sensory receptors located in the skin, muscles, tendons and joints) collectively work to coordinate movement with balance, and can also create illusory nonvisual sensations, resulting in spatial disorientation in the absence of strong visual cues.

In aviation, spatial disorientation can result in improper perception of the attitude of the aircraft, referring to the orientation of the aircraft relative to the horizon. If a pilot relies on this improper perception, this can result in inadvertent turning, ascending or descending. For aviators, proper recognition of aircraft attitude is most critical at night or in poor weather, when there is no visible horizon; in these conditions, aviators may determine aircraft attitude by reference to an attitude indicator. Spatial disorientation can occur in other situations where visibility is reduced, such as diving operations.

Spatial orientation in flight is difficult to achieve because numerous sensory stimuli (visual, vestibular, and proprioceptive) vary in magnitude, direction, and frequency. Any differences or discrepancies between visual, vestibular, and proprioceptive sensory inputs result in a sensory mismatch that can produce illusions and lead to spatial disorientation. The visual sense is considered to be the largest contributor to orientation.

While testing an early turn and slip indicator devised by his friend Elmer Sperry in 1918, United States Army Air Corps pilot William Ocker entered a graveyard spiral while flying through clouds without visual references; the turn indicator showed he was in a turn, but his senses told him he was in level flight. Emerging from the clouds, Ocker was able to recover from the dive. In 1926, Ocker was subjected to a Bárány chair equilibrium test by Dr. David A. Myers at Crissy Field; the resulting duplication of the somatogyral illusion he had experienced and a subsequent re-test, which he passed using the turn indicator, led him to develop and champion instrumented flight. Sperry would go on to invent the gyrocompass and attitude indicator, both of which were being tested by 1930. With Lt. Carl Crane, Ocker published the instructional text Blind Flying in Theory and Practice in 1932. Influential advocates of instrumented flight training included Albert Hegenberger and Jimmy Doolittle.

In 1965, the Federal Aviation Agency of the United States issued Advisory Circular AC 60-4, warning pilots about the hazards of spatial disorientation, which may result from operation under visual flight rules in conditions of marginal visibility. A new version of the advisory was issued in 1983 as AC 60-4A, defining spatial disorientation as "the inability to tell which way is 'up.'"

Statistics show that between 5% and 10% of all general aviation accidents can be attributed to spatial disorientation, 90% of which are fatal. Spatial-D and G-force induced loss of consciousness (g-LOC) are two of the most common causes of death from human factors in military aviation. A study on the prevalence of spatial disorientation incidents concluded that "if a pilot flies long enough ... there is no chance that he/she will escape experiencing at least one episode of [spatial disorientation]. Looked at another way, pilots can be considered to be in one of two groups; those who have been disorientated, and those who will be."

There are four physiologic systems that interact to allow humans to orient themselves in space. Vision is the dominant sense for orientation, but the vestibular system, proprioceptive system and auditory system also play a role.^{[citation needed]}

Spatial orientation (the inverse being spatial disorientation, aka spatial-D) is the ability to maintain body orientation and posture in relation to the surrounding environment (physical space) at rest and during motion. Humans have evolved to maintain spatial orientation on the ground. Good spatial orientation on the ground relies on the use of visual, auditory, vestibular, and proprioceptive sensory information. Changes in linear acceleration, angular acceleration, and gravity are detected by the vestibular system and the proprioceptive receptors, and then compared in the brain with visual information.^{[citation needed]}