Tohoku is the Japanese region where the city of Sendai resides. In reality, damage occurred within many parts of the Tohoku region, and Architecture for Humanity will be conducting projects around the whole area–not just Sendai. The Japan team is emphasizing that they are running a Tohoku rebuilding program, so we will be seeing this name more and more often.
You may also hear about the various prefectures within the Tohoku region. Sendai is located in the Miyagi prefecture, which rests just above the Fukushima prefecture.
Thanks Hiromi, Edutraveller.com and Japan-Guide.com for the info!