The number of HTML email recipients who opened your email, generally measured as a percentage of the total number of emails sent (measurements do vary). The open rate is considered a useful metric for evaluating response to an email campaign but it should be noted that open rates can be deceiving because if a recipient does not download images, an 'open' is usually not recorded.
An open relay is an email server configured (poorly!) so that anyone on the internet can send email. In the past this was an acceptable method for sending email, but now spammers use open relay to re-route their email through a third party to avoid detection. The CAN SPAM Act of 2003 made it illegal to send spam through an open relay. Most ISPs will not accept email from known open relays.
An opt-in subscriber is someone who expressly requests to receive emails from a person or company, thereby giving them permission to send them their email campaigns. In most cases an opt-in subscriber can edit their preferences to control the emails they receive.
An opt-out list or "Do Not Mail" list is a set of email addresses representing people who you cannot (and should not!) email. The opt-out list is typically comprised of people who have previously unsubscribed from your mailing list.
A person who chooses to opt-out or unsubscribe from your mailing list is expressly communicating that they do not want to receive future email messages from you. In EliteEmail.com, these people are added to your "Do Not Mail" list so that you do not accidentally make a mistake and send someone who previously opted-out another email.