和英辞書

国書

noun

a diplomatic letter, a message sent by the head of state / a piece of national literature; a literary work written in the national language, as opposed to a foreign language

彼はその国書を大統領に手渡した。

He handed the diplomatic letter to the president.

意味（1）

a diplomatic letter, a message sent by the head of state

その国(こく)書(しょ)の返(へん)書(しょ)期(き)限(げん)は約(やく)一(いち)年(ねん)先(さき)であったが、その間(あいだ)に国(こく)論(ろん)は、攘(じょう)夷(い)論(ろん)と開(かい)国(こく)論(ろん)に大(おお)きく分(わ)かれていこうとしていたのである。

Sono kokusho no hensho kigen wa yaku ichinen saki de atta ga, sono aida ni kokuron wa, jōiron to kaikokuron ni ōkiku wakarete ikō to shite ita no de aru.The deadline to reply to the diplomatic dispatch was about a year off, but during that time national opinion was undergoing a large schism into those favoring the exclusion of foreigners and those favoring the opening of the country.

意味（2）

a piece of national literature; a literary work written in the national language, as opposed to a foreign language

一(いっ)方(ぽう)で６(ろく)案(あん)の提(てい)示(じ)の仕(し)方(かた)や、事(じ)務(む)方(かた)による説(せつ)明(めい)の内(ない)容(よう)からは、「国(こく)書(しょ)を典(てん)拠(きょ)とした初(はつ)の元(げん)号(ごう)」を目(め)指(ざ)した形(けい)跡(せき)もみえる。

Ippō de roku an no teiji no shikata ya, jimukata ni yoru setsumei no naiyō kara wa, “kokusho o tenkyo to shita hatsu no gengō” o mezashita keiseki mo mieru.On the other hand, based on how the six candidates were presented and on the staff's explanation, it also appears that they were aiming for "the first era name sourced from national literature".