From the sci-fi movie Stargate and the television series Stargate SG-1, the Chaapa'ai is a piece of alien technology used to traverse great distances between thousands of different worlds throughout the galaxy. It is a large ring created from a rare, superconducting mineral known as Naquaada.
When the Chaapa'ai was first unearthed, the humans who figured out how to make it work called it the "Stargate." After they began traveling to other planets, however, the SG-1 team soon realized that everyone else knew it by its original name. (This is because, in the Stargate universe, all human life originated from Earth. See Tauri for an explanation.) For this reason, the Tauri (including the SG-1 team) consistently refer to the Chaapa’ai as the “Stargate,” while the Goa’uld, the Tok’ra, the Jaffa, and most other off-world beings refer to it by its original name. This is useful to the casual SG-1 viewer, as it’s pretty safe to assume anyone who calls the Stargate a "Chaapa'ai" probably ain’t from around here.
The Chaapa'ai works somewhat like a combination lock, with nine chevrons, or parameters, that are used to "dial out" and create a wormhole to other stargates. Usually, only seven of the chevrons are used to dial another planet; but in some cases (e.g. episode #215, The Fifth Race) an eighth chevron, acting as a type of "area code," may be employed. The purpose of the ninth chevron is, as of yet, unknown.