a shift is leaving the body in world A and transferring into a new body in world B. (however the new body may be completely identical to the previous body.)

A shift is roughly equal to the death of the shifter in world A.

The difference between a shift and a drift is that in drift, your body suddenly disappears from world A and appears in world B while in shifting, your previous body dies inside world A and then you will live inside your new body in world B.