(history, socialism) the Second International

second party, after-party

a quadratic equation

Used other than figuratively or idiomatically: see 二, 枚, 目. (the second sheet or other flat thing) / a handsome male character, especially one that gets involved in a romance during the story

(taxonomy) binomial nomenclature

(typography) the iteration mark 〻, used to repeat kanji, historically mostly kanji with kun'yomi

double room

(linguistics) doublet

going back to sleep (e.g. after waking up in the morning)

(idiomatic) unhesitatingly without dispute