Hard bounce
A permanent delivery failure — the address does not exist or refuses mail for good.
A hard bounce means the receiving server has rejected the message permanently: the mailbox does not exist, the domain does not resolve, or the recipient has blocked you outright. Retrying achieves nothing.
Hard-bounced addresses should be removed at once. Continuing to send to them tells the receiving side that you are not maintaining your list, which costs you deliverability for everyone else.
The counterpart is a soft bounce: a temporary failure such as a full mailbox or a server that is briefly unavailable. Those are retried automatically and only matter if they repeat.