Transformative AI is "[...] AI that precipitates a transition comparable to (or more significant than) the agricultural or industrial revolution."[1] Unlike the related terms Superintelligent AI and Artificial General Intelligence, which refer to specific capabilities the AI may have, Transformative AI refers to the effects the AI would have on humanity; our well-being, the global economy, state power, international security, etc.
Holden Karnofsky gives a more detailed definition in an OpenPhil 2016 post:
...[...] Transformative AI is anything that fits one or more of the following descriptions (emphasis original):