GTI stands for Grand Tourer Injection, which you'll also see in the Latin form Gran Turismo Iniezione. The name “Grand Touring“ insinuates a focus on performance, which you can definitely see in the Golf GTI, especially on the Autobahn trim. The “injection“ part of the acronym refers to the gasoline injection in the engine, which was a huge automotive innovation when it was first discovered. Since most engines are direct-injected now, however, it also is generally thought to point towards a more powerful engine.