Cyber-planets were planets that were dominated by the Cybermen. The Sixth Doctor identified Mondas as the Cyber-planet. (TV: Attack of the Cybermen) The Twelfth Doctor stated that, through parallel universes, the Cybermen began on various planets throughout time and space, including Mondas (and on a Mondasian colony ship), Telos, Earth, Planet 14, and Marinus. (TV: The Doctor Falls)
In 1988 the CyberIsomorphs attempted to transform Earth into a base planet, the New Mondas. (TV: Silver Nemesis)