Open rate refers to the percentage of recipients who open and view an email or newsletter that has been sent to them. It is a measure of the effectiveness of an email marketing campaign, as a higher open rate indicates that more people are engaging with the content. Open rate is typically calculated by dividing the number of emails opened by the total number of emails delivered, and is often used as a key metric in determining the success of an email marketing strategy. A high open rate can be achieved through compelling subject lines, relevant and personalized content, and proper targeting of the audience.