What does the abbreviation BCC stand for in an email header?

BCC in an email header stands for Blind Carbon Copy.

The term comes from carbon-copying paper correspondence: a carbon copy reproduced writing on another sheet, while a blind carbon copy was arranged so other visible recipients would not know who received it. In email, addresses placed in the Bcc field receive the message without their addresses being shown to recipients listed in the To or Cc fields.

Technically, Bcc recipients are still included in the message’s SMTP envelope through recipient commands such as `RCPT TO`. The Bcc header itself is normally removed or handled specially before delivery, so the visible message headers do not reveal the hidden recipients. Email clients and servers can implement details differently, so Bcc should not be treated as an encryption or absolute secrecy mechanism.

Bcc is useful for mailing lists, privacy-conscious group messages, and preventing recipients from exposing one another’s addresses. It can also reduce accidental “Reply All” storms. “Blind courtesy copy” is sometimes used as an alternative expansion, but “Blind Carbon Copy” is the original and canonical answer.