Release 27.0.0 is the first release in the Horizon 27 series.
It is a major enhancement release with many bug fixes and a ton of new features, most notably the removal of the Remote Poller in favor of a new Minion-based workflow called Application Perspective Monitoring.
For a high-level overview of what has changed in Horizon 27, see What’s New in OpenNMS Horizon 27.
The codename for 27.0.0 is Zaphod.