Chohan is a word that according to H. P. Blavatsky means "'Lord' or 'Master'; a chief". Although Mme. Blavatsky claims the word comes from the Tibetan language, its origin has not been identified. In the Mahatma Letter No. 18 the word is spelled as "Cho-Khan", the Tibetan words chos (pronounced with a silent "s") meaning "dharma" ("teaching", "doctrine", or "law") and khan (spelled mkhan) means abbot. Also, the word mkhan as the second member of a two-part word means "one who practices or is skilled in" something.
According to G. de Purucker
Chohan (Tibetan) [poss from chös law, dharma + Mong khan lord] “Lord of the dharma”; in The Mahatma Letters chohan is the title usually given to superiors among the Masters of the Great White Lodge, whose chief is called the Maha-chohan. Also a general term used for beings in several states of evolution higher than the human. “There are men who become such mighty beings, there are men among us who may become immortal during the remainder of the Rounds, and then take their appointed place among the highest Chohans, the Planetary conscious ‘Ego-Spirits’ ” (ML 130). Because chohan is used much as “chief” is used in English, the term does not signify one single degree in spiritual evolution.
According to C. W. Leadbeater
The title Chohan is given to those Adepts who have taken the sixth Initiation, but the same word is employed also for the Heads of Rays Three to Seven, who hold very definite and exalted offices in the Hierarchy. We are given to understand that the meaning of the word Chohan is simply “Lord,” and that it is used both generally and specifically, in much the same way as the word Lord is employed in England. We speak of a man as a lord because he possesses that title, but that is quite different from what we mean when we speak, for example, of the Lord Chancellor or the Lord-Lieutenant of the County.
According to Wikipedia,
Khagan or Qagan ... 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. 
- Chohan at Theosophy World