Rikkokushi; six national histories
Refers collectively to the six official national histories compiled in the Nara and Heian periods. Often used in historical contexts.
See also: 六国史
Nihongi is a general term for the six national histories compiled from the Nara to Heian periods.
Nihon Shoki; Chronicles of Japan
Specifically the second oldest work of Japanese history, completed in 720 CE. Also known as Nihongi.
See also: 日本書紀
The Nihon Shoki is the second oldest history book in Japan, completed in 720.
六国史 is the standard term for the six national histories; 日本紀 can refer to the whole set or specifically to the Nihon Shoki.
Formed from 日本 and 紀, “chronicle,” with voicing of 紀 to ぎ in the traditional reading. 日本紀 has been used as a title or abbreviated designation for the 日本書紀 and, more broadly, for the national histories.