Stalking is commonly defined as the willful, malicious and repeated harassing of an individual by another person. It is any repetitive, obsessive or unwanted contact or behavior that threatens or places fear in the victim, such as:
Following the individual
Showing up at her/his home, school, or workplace
Sending unwanted objects or messages (e.g. email, IM, Face Book, etc)
Making harassing telephone calls
“Stalking is an extraordinary crime, given that it may often consist of no more than the targeted repetition of an ostensibly ordinary and routine behavior.” (Sheridan, Blaauw, & Davies, 2003, p. 150)
In a survey, 13.1% of college women reported that they had been subjected to repeated, obsessive, and frightening behavior, which made them afraid or concerned for their safety in the previous 6-9 months. Of these stalking victims 80.3% knew or had seen their stalker before. In 15.3% of these incidents, the victim reported that the stalker either threatened or attempted to harm the victim. (Source: Fisher et al., 2000)
The Stalking Resource Center has more information.