Original: The number in 90 Antiope's name denotes that it was the 90th asteroid to be discovered. Its proper name comes from Greek mythology, though it is disputed as to whether this is Antiope the Amazon or Antiope the mother of Amphion and Zethus.
Simplified: The 90th asteroid to be found, it is named after Antiope from Greek mythology, though it is disputed as to whether the namesake is Antiope the Amazon or Antiope the mother of Amphion and Zethus.