What is the meaning of 関東?

Short for 関東地方 (Kantō-chihō): A geographical region in eastern Japan that includes the capital city of Tokyo and surrounding prefectures

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