Eta Cassiopeia (sometimes also referred to as Eta Cassiopeiae) is a binary star system located around 20 light-years from the Sol system. It was orbited by at least three planets, one of which became a Human colony in the late 21st century.
In 2087, the Terra Nova colony was established on the third planet of the system by Humans, in an attempt to prove that Humans could actually establish a colony in deep space. In 2178, the system became a member of the United Federation of Planets. (ENT episode: "Terra Nova" and ST reference: Star Charts)
During 2277, in the mirror universe, Grand Admiral Matthew Decker assembled a task force in this system in the hopes of ambushing the ISS Enterprise and killing Admiral Spock. However, mutinies aboard the starships loyal to Decker meant that Spock was able to seize control of the fleet and declare himself the Grand Admiral of the Imperial Starfleet in Decker's place. (TOS - Glass Empires novel: The Sorrows of Empire)