A bounce is an email message sent in response to an email message that cannot be delivered for some reason. The term refers to your original message “bouncing back” to you.
A message indicating that an email message has been delayed, but may still be delivered, is sometimes also incorrectly referred to as bounce. More accurately, the final message indicating that the message had not been delivered would be the bounce message.
Bounce messages are not guaranteed, meaning that not getting a bounce in response to sending an email message does not guarantee that the message has been delivered. Bounces themselves can be lost in the mail, or email providers can elect not to send them in some circumstances.