nounWord forms: plural Achaemenids, Achaemenidae (ˌækɪˈmɛnɪˌdiː) or Achaemenides (ˌækɪˈmɛnɪˌdiːz)any member of a Persian dynasty of kings, including Cyrus the Great, that ruled from about 550 to 331 bc, when Darius III was overthrown by Alexander the Great