From this law dictionary:

Harassment is governed by state laws, which vary by state, but is generally defined as a course of conduct which annoys, threatens, intimidates, alarms, or puts a person in fear of their safety. Harassment is unwanted, unwelcomed and uninvited behavior that demeans, threatens or offends the victim and results in a hostile environment for the victim. Harassing behavior may include, but is not limited to, epithets, derogatory comments or slurs and lewd propositions, assault, impeding or blocking movement, offensive touching or any physical interference with normal work or movement, and visual insults, such as derogatory posters or cartoons.

The following is an example of a state law dealing with harassment:

"S 240.25 Harassment in the first degree.

A person is guilty of harassment in the first degree when he or she intentionally and repeatedly harasses another person by following such person in or about a public place or places or by engaging in a course of conduct or by repeatedly committing acts which places such person in reasonable fear of physical injury. This section shall not apply to activities regulated by the national labor relations act, as amended, the railway labor act, as amended, or the federal employment labor management act, as amended.

Nice, thanks for the legal heads up.

So from what I'm understanding, harassment is really a question of point of view and while some might consider a situation as "mildly annoying", some might perceive it as "harassment".

To me, the key factors are repetition and organization: 1000 persons telling me once they think I'm an asshole on their own will would be mere proof of massive discontent while 1000 persons organized to fuck with me or a handful of people constantly fucking with me would be more akin to harassment.