In a narrow sense, it refers to the opening line of "Nihonshoki" (Chronicles of Japan): 'Long, long ago, when the heaven and the earth were not separated yet, and light and darkness were not distinguished...'
狭義には、『日本書紀』冒頭の「古（いにしえ）に天地未だ剖（わか）れず、陰陽分れざりしとき......」を言う。