(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

(linguistics) diphthong

2-step authentication

a digastric muscle

(grammar) The Classical Japanese bigrade verb conjugation class, consisting of the two subclasses of 上(かみ)二(に)段(だん)活(かつ)用(よう) (kami nidan katsuyō, “upper-bigrade conjugation”) and 下(しも)二(に)段(だん)活(かつ)用(よう) (shimo nidan katsuyō, “lower-bigrade conjugation”)

double-pane window, found especially in Hokkaido and Tōhoku as cold-proofing