Click-to-open rate (CTOR) is a metric that compares the number of unique clicks to unique opens for a given email.

This number is a good indication of how effective the subject line, images, design, and content performed.

Click-to-open rate is arguably a more helpful metric that just looking at the overall click-through rate, because it indicates, out of all the people who opened (and therefore saw) your email, how many then decided to click a link.

While it may sound similar, "click-to-open rate" is not to be confused with "open rate" or "click-through rate".