national unity; the whole nation acting as one
A yojijukugo strongly associated with wartime rhetoric, especially World War II, evoking the idea of 100 million people united in purpose. Rare in everyday modern speech outside historical or political commentary.
During the war, the slogan 'one hundred million hearts as one' rallied the nation.
A yojijukugo composed of 一億 (one hundred million, a figure historically used for Japan's population) and 一心 (one heart/mind). The exact origin is uncertain, but it became a prominent wartime slogan.