Rashi: It is a Kli for letters and Seforim. It is from the root Chemas [Mayim]; the Aleph is not from the root, like other Arame'ic words [that have an extra Aleph at the beginning - refer to 1:9:1:3, 3:3:3:1*].

Ibn Ezra: It is a Kli in which chronicles are hidden.

Rashi: In the capitol.

Ibn Ezra: It is a castle. The Targum of "b'Armenoseinu" is b'Varnisana.

Malbim: Achashverosh moved the capitol to Shushan. Then, the command was lost. Perhaps Haman destroyed it, for it benefited the Jews! It was not found in Bavel. Via Hashgachah, they found in Madai a remembrance of the command, in chronicles that were not in Haman's hands.

Rashi: Was written in it the remembrance [of the command].