Earth (Terra in Ancient, Midgard to the humans settled by the Asgard, and Tau'ri in Goa'uld) is the home planet of humanity and the cradle of human civilization in the Milky Way. Although it languished in obscurity and barbarism for many thousands of years after the System Lords abandoned the region, Earth would later rise to prominence when its native population rediscovered their stargate. Today, the planet is regarded as one of the most-influential and prosperous in the galaxy, and serves as a major outpost of the Interstellar Alliance.
Earth was among the first planets that the Ancients colonized when they left their home galaxy. Some five million years ago, the Ancients were forced to leave in Atlantis for Pegasus due to a plague. They eventually returned after the war with the Wraith.  
Shortly after all the Ancients had ascended, leaving the remaining humans in a primitive and defenceless state, the Goa'uld known as Ra, discovering Earth and its inhabitants, took it for his own and ruled for 5,000 years. An alternate SG-1 travelled back in time and helped the ancient humans rebel against Ra, who in turn abandoned Earth. 
Thanks to the rediscovery of the stargate, the Tau'ri have come to find out that Humans live on other planets in addition to Earth, each with their own unique cultures. As the Stargate Program progresses, the term Tau'ri eventually came to be used to specifically identify humans from Earth.
In recent years, the planet has fallen under siege numerous times, as its rapid rise to prominence has attracted the attention of many enemies; the System Lords all failed to conquer it, the Ori tried and failed during their 3-year crusade, and the Der'kal, while successful in finally taking the planet, were forced to abandon it less than two years later, unable to prevent the humans from retaking their homeworld. Prior to this latest event, and now slowly returning after it, Earth had gained a reputation as being an "invincible planet," a stronghold which no power could hope to defeat due to the strength of its native people.
- In an alternate reality Earth was attacked by Apophis and most major cities were destroyed. Efforts to combat the invasion were headed in Cheyenne Mountain by the SGA. Samantha Carter stated that Washington D.C. and Philadelphia had been lost. Cities that, based on the SGA's map, were almost certainly destroyed include the following: Cairo, London, Moscow, New York City, and Paris. Colorado Springs was presumably destroyed during the attack on Cheyenne Mountain. The Goa'uld attack started in Europe, Asia and Africa before spreading to the United States. Since the destruction started on the East Coast and moved west and Daniel Jackson left the reality when it reached Colorado, the annihilation of the West Coast was unseen. 
- In an alternate reality Earth was attacked by Apophis and the SGC was invaded. Earth was saved when Samantha Carter successfully contacted the Asgard. 
- In the alternate timeline created by Ba'al, Earth was attack by the entire Goa'uld fleet. 
- In an alternate reality, Earth came under attack by a Hive ship, but it was destroyed by the Ancient weapons platform. A single Wraith survived and was later killed, but not before sending a message that was picked up by the ZPM powered hive in our universe, telling it the location of Earth and about the weapons chair. 
Behind the scenesEdit
While never given a planetary designation during any of the on-screen material—other than "Earth" or "Tau'ri"—Earth is identified as P2X-3YZ in the Secrets Files of the SGC bonus features on the Stargate SG-1 DVD's. In these features, the "viewer" is said to be viewing top secret SGC files which give the viewers location as being P2X-3YZ.