A wheel or disk mounted to spin rapidly about an axis and also
free to rotate about one or both of two axes perpendicular to each
other and the axis of spin so that a rotation of one of the two
mutually perpendicular axes results from application of torque to the
other when the wheel is spinning and so that the entire apparatus
offers considerable opposition depending on the angular momentum to any
torque that would change the direction of the axis of spin.
-- Webster's Seventh New Collegiate Dictionary