The final and decisive engagement of the Earth-Romulan War in 2160, in which Earth and its allies inflicted a defeat that the Romulans still remembered as a humiliation two centuries later.
The Battle of Cheron has never been shown on screen. It is known from The Next Generation episode The Defector, in which the Romulan admiral Alidar Jarok says that a new war would force his people to avenge their humiliating defeat there.
The battle brought the Earth-Romulan War to an end. As Spock recounts in the Original Series episode Balance of Terror, that war was fought with primitive atomic weapons and without visual contact, and the peace that followed was negotiated by subspace radio and established the Romulan Neutral Zone.
The alliance that won the war went on to found the United Federation of Planets in the following year. Cheron here is unrelated to the planet of the same name seen in the Original Series.