The phrase '八紘一宇' contains the idea of uniting the world as if under one roof.
A historical slogan meaning 'the whole world under one roof'; 一宇 is the 'one roof' component.
Compound of 一 (いち, 'one') and 宇 (う, 'eaves; roof; house'). The exact historical derivation is uncertain; the term is primarily known through the 20th-century slogan 八紘一宇.