'Letters' in the Bible

Further, I said to the king, If it is the king's pleasure, let letters be given to me for the rulers across the river, so that they may let me go through till I come to Judah;

Then I came to the rulers of the lands across the river and gave them the king's letters. Now the king had sent with me captains of the army and horsemen.

And further, in those days the chiefs of Judah sent a number of letters to Tobiah, and his letters came to them.

And they said much before me of the good he had done, and gave him accounts of my words. And Tobiah sent letters with the purpose of causing me fear.