Terra

Galactica in orbit of Terra.

Planet Terra is home to the Terran Nationalists and the Eastern Alliance. The Eastern Alliance would have eliminated the Nationalists in a nuclear attack if not for the intervention of Apollo and the Galactica. Despite hopes that Galactica had finally reached its destination, it is revealed that Terra is not Earth (Experiment in Terra).

Colonists from Terra founded the Lunar Colonies. The Galactica first encounters "Earthlings" when they discover a ship from the Lunar Colonies. The people aboard were attempting to flee the influence of the Eastern Alliance by travelling to Paradeen (Greetings from Earth).

The Link to Earth

In the episodes Greetings from Earth and Experiment in Terra there are clues pointing to the people of Terra being descendents of the Thirteenth Tribe:

The names of Terrans are Earthling names (e.g. Michael, Barbara), with a first and last name that is consistant with a great deal of Earth's cultures.

Terra is Latin (and Gemonese) for Earth.

The Terrans have never heard of centons as the Terran Michael asks Apollo what a centon is.

Original Series definition: a unit of time akin to a "minute"; in earlier episodes (e.g. "Saga of a Star World," "Lost Planet of the Gods, Part I"), centon is used as the Colonial equivalent of a week before being retconned in later episodes