Deviation course is a term used in navigation to describe the intentional change of direction from the original course. YAW is a three letter word that describes the rotation of an aircraft or ship around its vertical axis. Driftage is an eight letter word that describes the unintentional deviation of a vessel from its intended course due to external forces such as wind and current. Together, these two terms describe the intentional and unintentional changes in direction of a vessel during navigation.