What are the terms for an unmarried woman?

A woman who has never been married is a spinster and addressed as miss.

A woman who has been married but who's husband has died is called a widow and addressed as misses (mrs).

A woman who has been married but is divorced is called a divorcee. Historically, a divorcee would use her own first name formally (eg Mrs Jane Doe) a woman who was still married would use her husband's name (eg Mrs John Doe).