General synonyms:
* Abuse
* Bullying
* Intimidation
* Persecution
* Molestation (often used in a sexual context)
* Vilification
* Badgering
* Tormenting
* Pestering
* Hounding
* Stalking (when the harassment involves following or watching someone)
More specific synonyms:
* Sexual harassment
* Racial harassment
* Religious harassment
* Workplace harassment
* Cyberbullying
* Cyberstalking
The best synonym to use depends on the specific situation and the type of harassment being described.