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gimbal lock[′gim·bəl ‚läk]
A condition of a two-degree-of-freedom gyro wherein the alignment of the spin axis with an axis of freedom deprives the gyro of a degree-of-freedom and therefore its useful properties.
A situation in which two gimbals coincide in the same plane. This results in the restriction on the freedom of movement of a gyroscope, robbing it of its useful properties.