(blind carbon copy) used in an email when you are sending a copy of a message to various people, when you do not give a complete list of all the people you are sending the message to 〔电子邮件的〕密件抄送，密件副本
From Longman Business Dictionary
bccbcc noun [countableC]COMPUTING
(blind carbon copy) used in an email when sending someone a copy of a message that you have also sent to someone else, so that this person does not know that other people will also receive the message