A hard bounce is a term used in email marketing to describe the failure of an email to be delivered to its intended recipient. This occurs when the email is returned to the sender due to a permanent error, such as an invalid or non-existent email address. Unlike a soft bounce, which is a temporary delivery failure, a hard bounce indicates a permanent issue and the email will not be able to be delivered to that address in the future. Hard bounces can negatively impact email deliverability and should be monitored closely by email marketers to maintain a healthy email list.