Concise Bible Dictionary:

1. Father of Darius, incidentally named in Daniel 9:11In the first year of Darius the son of Ahasuerus, of the seed of the Medes, which was made king over the realm of the Chaldeans; (Daniel 9:1). It is supposed that the word Ahasuerus is an appellative, or official title, as Pharaoh was in Egypt, and that the person referred to is the Cyaxares of history, king of Media.
2. Persian king to whom the enemies of the Jews made their accusation against those in Judah and Jerusalem (Ezra 4:66And in the reign of Ahasuerus, in the beginning of his reign, wrote they unto him an accusation against the inhabitants of Judah and Jerusalem. (Ezra 4:6)). He is supposed to be Cambyses, son of Cyrus.
3. Persian king who “reigned from India even unto Ethiopia,” and took the Jewish maiden Esther to be his queen. He is held to have been Xerxes, son of Darius Hystaspis. This was in the seventh year of his reign, the year when he returned from his unsuccessful expedition against Greece. His rash conduct in repudiating his queen because she would not violate the decorum of her sex, and his giving up the whole of the Jewish people to the pride of Haman, agrees with his folly in scourging the sea and putting the engineers to death because a storm injured the bridge they had made. See ESTHER. For the succession of the Persian kings see PERSIA.

Strong’s Dictionary of Hebrew Words:

or (shortened) pAchashrosh {akh- ash-rosh'} (Esth. 10:1); of Persian origin; Achashverosh (i.e. Ahasuerus or Artaxerxes, but in this case Xerxes), the title (rather than name) of a Persian king
KJV Usage:

Jackson’s Dictionary of Scripture Proper Names:

I will be silent and poor

Potts’ Bible Proper Names:

A prince clothed with majesty:―name of one Median and two Persian kings [ARTAXERXES], Esth. 1:1. {Majestate praeditus princeps}