The Ancients who originally inhabited the city of Atlantis. The Lanteans brought the city from Earth to the Pegasus Galaxy several million years ago and seeded human life there. After many eons (and a war with the Wraith, which they helped to create) the Lanteans decided to leave the city and the galaxy behind, sinking their great city beneath the oceans of Lantea and traveling through the Stargate to Earth some 10,000 years ago.
The name is likely an abbreviation of "Atlanteans," and may be Pegasus Galaxy slang. The term is used by the Wraith, among others.
See also: Ancients
The Defiant One - A Wraith commander recognizes a Puddle Jumper to be Lantean technology, but believes that his race defeated the people long ago.
Before I Sleep - Dr. Weir travels 10,000 years into the past and meets Atlantis's last inhabitants, who submerge the city and gate to Earth.
The Return, Part 1 - The team finds an Ancient warship travelling home, and helps them return to Atlantis -- only to see them expel the team from Earth from their adopted home.