Wiktionary
n. A ''khan of khans'', a ruler of a khaganate; (a holder of) an imperial rank used among certain Turkic and Mongolian peoples, equal in status to an emperor.
Wikipedia
Khagan or Qagan (, Khaan, Mongolian Script: , Qaγan; ; ; , Khāqān, alternatively spelled Kagan, Khaghan, Kha-khan, Xagahn, Qaghan, Chagan, Қан, or Kha'an) is a title in the Mongolian language equal to the status of emperor and used to refer to someone who rules a khaganate or empire. The title was adopted by Ögedei Khan from the Turkic title kaɣan.
It may also be translated as Khan of Khans, equivalent to King of Kings. In modern Mongolian, the title became Khaan with the 'g' sound becoming almost silent or non-existent (i.e. a very light voiceless velar fricative); the ğ in modern Turkish Kağan is also silent. Since the division of the Mongol Empire, emperors of the Yuan dynasty held the title of Khagan and their successors in Mongolia continued to have the title. Kağan and Kaan are common Turkish names in Turkey.
The common western rendering as Great Khan (or Grand Khan), notably in the case of the Mongol Empire, is translation of Yekhe Khagan (Great Emperor or Их Хаан).
Usage examples of "khagan".
If he had hoped Wulghash, the khagan of Yezd, would send out some half-barbarous chieflet, he saw at once he was to be disappointed.
He loathed the style, but his sovereign the khagan insisted on it as a reminder that the Khatrisher ruling class ultimately sprang from Khamorth stock.
Viridovix turned his head to see Arghun the khagan and his younger son Dizabul coming up alongside the guardsmen.
The khagan obeyed without question, and did not draw back when the shaman cut his forefinger.
Royal Khagan, he would not go back to outlawry, the best fate he could expect from failure.
The khagan sat on the right-hand one, which was higher than the other.
Seeing him scratch his head, the khagan realized his mistake and repeated the request in his own language.
Wulghash might have heard of Romans from Avshar or from the spies the khagan had to have in Videssos.
But the senior centurion revealed nothing, for the khagan had slightly misread his man.
The plainsmen needed it to trade their tallow, their honey, their wax, their furs and slaves for wheat, salt, wine, silk, and incense from Videssos, but many a nomad khagan had coveted it for his ownand so the stonework.
Before long, no khagan in his right mind would have invited Varatesh to join him and his.
But when the khagan died, his sons quarreled, and the nomads went back to living clan by clan.
Until their khagan chose one side or the other, the Arshaum were carefully keeping Bogoraz and his retainers separate from the Videssian party.
But he quickly regained the advantage when he spoke to the khagan in the Arshaum tongue.
Repeating his half bow to the khagan, the Greek tried to say that in the Arshaum speech.