A special Blueprint that continues to affect any page that uses it.
A Live Blueprint is a special Page Blueprint that maintains a connection to any page that uses it. This means that you can make a change to the source Live Blueprint and then any of the pages on which it is used will also see the change. In this way, there is a persistent and live connection from these pages back to the source Blueprint.
This live connection is what makes a Live Blueprint different from a Page Blueprint or Space Blueprint. These latter two can be thought of as the stamp from which copies can be stamped out. Changes to the stamp will only affect future copies. Whereas a Live Blueprint, by contrast, might best be thought of as a link or a reference. Changes to the source Live Blueprint automatically update any place it is referenced.
Like Page Blueprints and Space Blueprints, though, you are able to use substitution fields in your Live Blueprint. Indeed, that's what makes them particularly valuable (and quite different from Confluence's Include Page macro).
Live Blueprints are not accessible from the Blueprints Menu. They are only able to be used on a page with the Live Blueprint Macro.