Nolo's Plain-English Law Dictionary
Repeatedly and obsessively harassing someone in ways that alarm and frighten them. Commonly, stalking begins with annoying or threatening phone calls, letters, or electronic communications and escalates to constant following or surveillance. Stalking is a crime in every state, though it may be called by another name, such as criminal harassment. It can be difficult to convict someone of stalking because much of the behavior may be legal when looked at in isolation; only when acts are repeated intensively do they become intimidating and abusive, rising to the level of illegal stalking.