This is defined as engaging in:
willful, malicious and repeated following or harassing another with credible threats with the intent to place person in reasonable fear of death or bodily injury
A few observations:
1. George Zimmerman is one of those rare aggravated stalkers who notifies the police before he commits his crime.
2. According to what he tells the dispatcher, it is Trayvon Martin who first approaches him. Only when Martin runs away does Zimmerman follow. He does so between about 2:14 (when his truck door slams) to about 2:29 on the tape, when he says "OK" in response to the advice not to follow the teen. Even if the "OK" doesn't mean he complies, by 2:39 Martin has apparently disappeared. (Zimmerman says "he ran" instead of "he's running.") The aggravated stalking goes on for about 15 or 25 seconds, and is not very successful.