Cybertron is the home planet of the Cybertronian race.
History
Millions of years before the 23rd century, Cybertron was a peaceful world that enjoyed a "Golden Age" before the planet became consumed by the Great War, its third world war, between the Autobots and the Decepticons. Four million years later, Cybertron had become a rogue planet and had drifted into the Sol system and risked being destroyed by the Sol asteroid belt. To save the planet, Optimus Prime launched a spacecraft that blasted a path through the rocks only for Megatron's ship to attack his own, sending both of their vessels crashing into the nearby Earth. (TAS - Star Trek vs. Transformers comics: "Issue 2", "Issue 3")
On stardate 5892.7, Starscream claimed that all the Cybertronians in the Milky Way Galaxy were located on Cygnus Seven, suggesting that Cybertron lay beyond the Milky Way. (TAS - Star Trek vs. Transformers comic: "Issue 4")