I can remember well that when I started learning Thai Massage in 2009, I thought that Ajarn was the first name of Ajarn Pichest Boonthumme. Pichest, by the way, is a famous Thai Massage teacher in the Chiang Mai region in Thailand and usually referred to as Ajarn Pichest.
It was only later that I understood that Ajarn is a Thai word ( อาจารย์ ) derived from the Indian Buddhist Pali Acariya that means respected Teacher, Instructor or Professor. But then I also discovered that Ajarn could be written as Ajaan, Aajaan, Ajarn, Ajahn, Acharn or Achaan, to name the most common phonetic conversions of the Thai script to the romanized alphabet.
Thus, Ajarn is a title that can be applied to any kind of teacher in Thailand, may that be for a Muay Thai (Thai Boxing) instructor, a learned Buddhist Monk, an elementary school teacher, or professor at a university, and so on. In Thailand, it’s common to address teachers with Ajarn before their first or last name. It’s a token of respect.
You will also encounter the title Khru for Thai Massage teachers. Khru is the Thai form of the Indian Sanskrit word Guru which also means Teacher.
The word Khru is well-known in the context of the Wai Khru ceremony, which is a widespread ritual in Thailand to say thanks to all your teachers going back in time up to paying respect to the Lord Buddha. Wai Khru ceremonies are performed within the Muay Thai, Krabi Krabong, Thai Traditional Dance, Thai Massage, musicians, doctors, soldiers, and regular school communities, to name just some examples.
At Thai Massage schools in Thailand, you would also see the teachers themselves performing the Wai Khru early in the morning before the classes start. They also, say thanks for the knowledge transferred to them by the Buddha, Dr. Jivaka (the supposed Father and Founder of Traditional Thai Medicine), and their own direct teachers and lineage.
Now and again, you will also hear the word Khun, which is a common Thai honorific title. Thai given names, out of politeness, are often preceded by Khun (Mr. Mrs. or Miss), so you will often see this with female Thai Massage teachers. Yet, the Khun title can be used for men also.
Then in past years, we increasingly see Thai Massage teachers addressed (or addressing themselves on their websites) by the title “Master.” This, of course, is English, but for many foreign students it has more familiarity, and subsequently it makes perhaps more sense. It’s also simply a translation of the aforementioned Ajarn honorific title.