Wiktionary
n. The loss of one degree of freedom in a three-dimensional space that occurs when the axis of two of the three gimbals are driven into a parallel configuration, "locking" the system into rotation in a degenerate two-dimensional space.
Wikipedia
Gimbal lock is the loss of one degree of freedom in a three-dimensional, three- gimbal mechanism that occurs when the axes of two of the three gimbals are driven into a parallel configuration, "locking" the system into rotation in a degenerate two-dimensional space.
The word lock is misleading: no gimbal is restrained. All three gimbals can still rotate freely about their respective axes of suspension. Nevertheless, because of the parallel orientation of two of the gimbals' axes there is no gimbal available to accommodate rotation along one axis.
Usage examples of "gimbal lock".
Capcom, please inform the crew that it needs to maneuver out of a threat of gimbal lock.