What term is used to describe the practice of adding a hidden recipient to an email?

The term that describes the practice of adding a hidden recipient to an email is Bcc, which stands for "Blind carbon copy." When an individual is added to the Bcc field of an email, their email address remains hidden from all other recipients. This allows the sender to send the same email to multiple people without disclosing the list of addresses to each recipient, enhancing privacy and discretion. Using Bcc is particularly useful in situations where the sender wants to avoid revealing all recipients’ information, such as in large mailing lists or when confidentiality is important.

The other terms mentioned do not serve the same purpose. Cc, or "carbon copy," allows recipients to see who else received the email, whereas the subject line identifies the email's topic. An attachment refers to additional files included with the email, not to the method of sending to hidden recipients.