It can also be a term that describes a person who wishes to be in a relationship with another person, gets close to that person as time goes on but never actually ask her/him out for some reason
Usually they quit after the person gets into a relationship but in some cases they extend to infinity
can become stalkers
in some cases the other person realize their affection, which is their goal, and result in happy ending
Person A: Al is always hanging out with Christina these days
Person B: yeah Al is definitely into her
Person A: Did he tell her yet?
Person B: no he is too shy
Person A: man that is tough. how long is he going to keep this up?
-ten years later at Allison's wedding
Person A: how's Al doing these days
Person B: pretty good. look he's best man
Person A: wow he is one tough asymptote