Term coined by Nicholas Ballard to describe gargantuan beings that dwell within a lepton-rich, thousand-meter-high pyramid on P7X-377. The beings appear to originate from within the depths of the interior of this pyramid, which Ballard believed to drop forever when he threw his torch over a ridge.
Though there may be others, one giant alien which exists in the pyramid calls himself Quetzalcoatl -- likely connected to the ancient Mayan deity on Earth -- and speaks in Mayan as well as English. The beings appear to be gaseous in form, and use the pyramid's high concentration of neutrinos (in conjunction with a Crystal skull) in the pyramid to make themselves visible to humans.
Only those who have been enveloped by the high radiation emanating from the crystal skull are able to see and hear the beings. They are enemies of the Goa'uld, and their technology does not allow Goa'uld or Jaffa to see them. Not only does this determine friend or foe, but it also provides some degree of protection for the alien beings. Their opening greeting tends to be "The enemy of my enemy is my friend," in Mayan.
The giant aliens were first encountered by Ballard after he found a skull in Belize, South America in 1971. When SG-1 found a similar skull on P7X-377 and brought Ballard there through the Stargate, the aging archaeologist -- Daniel Jackson's grandfather -- elected to remain behind with the beings and to learn from them.
Crystal Skull - Nicholas Ballard revisits the giant aliens, along with SG-1, and agrees to remain with them to learn from them, and they from us, as a sign of future cooperation and friendship.