a system where Certificate Authorities are required to log all certs they issue to one or more Certificate Transparency Logs, which are publicly accessible. The purpose is to add additional accountability for CAs, and allow easy searching of all certificates issued to a domain.

Certificate Transparency is a system where Certificate Authorities are required to log all certs they issue to one or more Certificate Transparency Logs, which are publicly accessible. The purpose is to add additional accountability for CAs, and allow easy searching of all certificates issued to a domain.

CT was brought in partly to make it easier to catch CAs intentionally issuing fraudulent certs, and partly to allow web site administrators to monitor when a cert is issued to their domain that they did not request. Google maintains a Transparency Report page where you can search the logs for all certificates issued to a particular domain:

Google Transparency Report

This is a project introduced by Google, and they still run the project.