From AMS Glossary
The drift of a tropical cyclone through the large-scale average layer-mean background wind in which it is embedded.
The drift is caused by the advection of the background potential vorticity field by the storm circulation. In the simplest case, the background potential vorticity gradient is simply the meridional gradient of the Coriolis parameter, β, from which the term gets its name. Beta drift generally causes tropical cyclones to move poleward and westward relative to the motion they would have if the background potential vorticity field were unperturbed by the storms. This drift speed is generally around 1–2 m s-1.