noun
Tokyo and three surrounding prefectures
Refers to the greater Tokyo area: Tokyo, Kanagawa, Chiba, and Saitama. Often used in news, statistics, and regional planning contexts.
一都三県で緊急事態宣言が発令された。
A state of emergency was declared in Tokyo and the three surrounding prefectures.
一都三県の人口は日本の約3割を占める。
The population of Tokyo and the three neighboring prefectures accounts for about 30% of Japan's total.
首都圏 is a broader term for the Tokyo metropolitan area, often including more prefectures; 一都三県 specifically names the four core administrative divisions.
Compound of 一都 (one metropolis, i.e., Tokyo) and 三県 (three prefectures: Kanagawa, Chiba, Saitama). A modern administrative and media coinage.