The definition of harassment often varies, depending on the context in which it happens. It can appear in many forms, some better defined than others, not all of which are always easy to detect. In the context of online interactions, harassment is generally considered as the act of systematic and/or continued, unwanted and annoying actions of an individual or a group, towards another individual or group, conducted outside of accepted societal and/or community norms.
Throw the element of fear into the mix [the intent to cause fear to the affected individual(s) through the aforementioned behaviour], and you have upgraded to one specific form of harassment: stalking. This [the element of fear] can also be what distinguishes criminal harassment from civil harassment, although this can vary from one country/state to the next, depending on statutes.
Harassment can manifest in multiple different ways. In the wiki-universe, this may include but is not limited to: