Refer to figure.
Any departure of a gyro axis from its original orientation is called wander. Wander may be either real or apparent (the difference is explained below) or a combination of both. Wander is subdivided into drift and topple. If the gyro axis wanders in the horizontal plane, it is called drift, either real or apparent.

APPARENT WANDER
In this type of wander, the gyro spin axis does not physically wander away from its pre-set direction, but to an observer it appears to have changed its direction.

This is because the gyro maintains its direction with respect to a fixed point in space, whereas the observer rotates with the Earth, thus with the passage of time the gyro appears to have changed direction, with reference to an Earth datum.

Apparent wander, is also made up of horizontal components called drift, and vertical components called topple. The rate of drift and topple depends upon the latitude and can vary from zero to a maximum of 15.04° per hour (the rate at which the Earth rotates).