What is the definition of whither?

It is the counterpart of "where". "Where" asks the location of something which is immobile; "whither" asks the destination of something which is moving. "You have captured and bound the prisoner; where are you keeping him?" but "The prisoner has escaped; whither is he running?" It is most often replaced in current English by "to where" or "where to"

The third word in this series is "whence" which asks the point of origin of a moving object. "We have captured an escaped prisoner; whence did he escape?" It is most often replaced by "from where" or "where from".