the whole world; the entire universe
A yojijukugo (four-character compound) meaning the entire world or universe, often used in Buddhist or literary contexts to express vastness or totality.
I pray for peace throughout the whole world.
This teaching spread across the entire universe.
世界 is the common word for 'world' in everyday use, while 一天四海 is a formal, literary yojijukugo with a grander, more universal scope.
宇宙 is the standard term for 'universe' or 'outer space'; 一天四海 is a more poetic, all-encompassing expression.
A yojijukugo composed of 一 (one), 天 (heaven), 四 (four), and 海 (sea), literally 'one heaven, four seas'. The phrase originates from Buddhist cosmology, where it refers to the entire world or universe under one heaven and surrounded by four seas.