The EEC has been working on standardizing metrics used in email marketing. They have published a set of definitions for different terms many email marketers use. They published their Support the Adoption of Email Metrics (S.A.M.E) guide in June.
Under the new EEC definitions an open is measured when either a tracking pixel is displayed or a user clicks on any link in the email, including the unsubscribe link. Open rate is defined as the number of opens (either unique or total) divided by the number of accepted emails. Accepted emails equals the number of emails sent minus the number of emails rejected by the ISP for any reason.
The authors do caution, however, that even their measurements may under count the number of email subscribers that actually open or read an email. Some readers don’t load images or click on links but happily read and digest the content being sent. Others may not click on a link but actually visit a website or brick and mortar store to purchase something based on the email.
Overall, I think the definitions created by the S.A.M.E. group accurately reflect the things they want to measure within the limits of what is actually measurable. Their definitions won’t affect conversations in the short term, but are likely to drive change to standard terminology over the longer term. I do strongly encourage people to grab a copy of their document and see how their definitions compare with your current measurements.