Stalking

Stalking is to follow or observe (a person) persistently, sometimes done out of obsession or derangement.

Examples are a fan stalking a celebrity, somebody stalking an ex-lover who he or she wants back, and other cases of non-reciprocal love.

The legality of the practice is defined differently under different jurisdictions, some define the act as it stands as illegal, some only define stalking illegal once it becomes threatening or endangers the one being stalked.