A psychic message left on the planet Aia hundreds of thousands of years ago, which the Romulan Zhat Vash read as a warning that synthetic life would bring about the destruction of all organic civilisation.
The Admonition is a store of memories left on Aia, a world the Romulans call the Grief World, which orbits within an artificially arranged system of eight suns. It was created some two to three hundred thousand years before the 24th century.
The Romulan cabal known as the Zhat Vash found it and made experiencing it an initiation rite. Most who endured the vision were driven to madness or suicide; the survivors, among them Commodore Oh, Narissa and Ramdha, devoted themselves to wiping out artificial life. The rite is shown in the Picard episode Broken Pieces.
In Et in Arcadia Ego the synthetics of Coppelius learn that the message was in fact addressed to them, a promise of aid from a distant alliance of machine intelligences.