Learn in the textbookHow Email Works and Specifying Recipients
Definitionbcc (blind carbon copy) is the address field for adding people without telling the other recipients, and those addresses never reach anyone in To or cc.
ExplanationPeople who receive a message through To or cc cannot tell that anyone is in bcc. It is a common way to keep addresses from getting out when mailing a large group of people who do not know each other's addresses.
AnalogyLike quietly passing a copy of a circulated document to one more person without telling the others.
Memory hookThe b stands for blind: read it as a cc kept out of the other recipients' sight.