1.
prime minister, chancellor, chief minister
A formal or literary term for the head of government or chief minister. In modern Japanese, used as an elevated synonym for prime minister, or to refer to historical chancellors and chief ministers.
Usage
More literary and formal than 首相 (prime minister) or 総理大臣. In modern journalism, 宰相 is sometimes used for stylistic variation or to lend gravitas. It also appears when discussing foreign heads of government, particularly historical ones.
Historically, the term referred to the highest-ranking minister in Chinese and Japanese imperial courts.