Bcc: Blank carbon copy to tertiary recipients who receive the message. The primary and secondary recipients cannot see the tertiary recipients. Depending on email software, the tertiary recipients may only see their own email address in Bcc, or they may see the email addresses of all recipients.
It is common practice to use the Bcc: field when addressing a very long list of recipients, or a list of recipients that should not (necessarily) know each other, e.g. in mailing lists