‘Sì hǎi’ literally means ‘four seas,’ a classical Chinese expression rooted in ancient cosmology. In pre-modern China, the ‘four seas’ referred symbolically to the vast waters surrounding the known world — not actual geographic oceans — and came to represent ‘all under heaven,’ i.e., the entire civilized realm or the whole world as understood by traditional Chinese thought.

This term appears frequently in classical texts, poetry, and formal speech to convey universality, inclusiveness, or broad scope. It carries a dignified, literary tone and is rarely used in casual conversation. While ‘sì’ (four) and ‘hǎi’ (sea) are concrete words individually, together they function as an idiomatic noun meaning ‘the world at large’ or ‘every corner of the land.’ It often appears in fixed phrases like 四海之内 (within the four seas = within the realm) or 四海升平 (peace throughout the world).