the daytime, during the day / a half day, half of a day

the eighth day of the month / eight days

the ninth day of the month / nine days

the seventh day of the month / seven days

フッ素, 弗素: fluorine

Short for ハイソックス (hai sokkusu, “[knee-]high socks”).

ソ連: the Soviet Union

historian

study of history

(Internet slang, especially anime and manga fandom) events or things from the past which one wants to whitewash/erase from history (due to it being embarrassing, poorly received, etc.); dark past