What is the meaning of 関東?

Kantō (a geographical region in eastern Japan, that includes the capital city of Tokyo and the surrounding prefectures of Chiba, Gunma, Ibaraki, Kanagawa, Saitama and Tochigi)

the area east of 逢坂の関 (Ōsaka-no-seki, tollgate of Ōsaka-yama) or later, east of 箱根の関 (Hakone-no-seki, tollgate of Mount Hakone)

the area to the east of 函谷関 (Kankoku-kan, Hangu Pass)

the area to the east of 山海関 (Sankai-kan, Shanhai Pass)

a surname

the shogunate

NHK Broadcasting Culture Research Institute, editor (1998), NHK日本語発音アクセント辞典 [NHK Japanese Pronunciation Accent Dictionary] (in Japanese), Tokyo: NHK Publishing, Inc., →ISBN

Source: wiktionary.org