Ajahn (Thai: อาจารย์, RTGS: achan, IPA: [ʔāː.tɕāːn], also romanized ajaan, ajarn, acharn and achaan) is a Thai language term which translates as "teacher". It is derived from the Pali word ācariya, and is a term of respect, similar in meaning to the Japanese sensei, and is used as a title of address for high-school and university teachers, and for Buddhist monks who have passed ten vassa.

This lattermost case is usually romanized ajahn. According to the Vinaya, any properly ordained monk can become an ācariya after ten vassa in the robes, thus a Thai monk becomes ajahn.

A senior monk may bear the honorific title phra ajahn (Thai: พระอาจารย์,"venerable monk"), or in more informal situations, than ajahn (Thai: ท่านอาจารย์,"venerable monk").

Some famous ajahns are:

In Thai, such highly esteemed monks would very rarely be called simply ajahn chah, ajahn mun, etc., as there are much more respectful ways for addressing or referring to them.

Ajahn Brahmavamso Mahathera (known to most as Ajahn Brahm), born Peter Betts in London, United Kingdom on 7 August 1951, is a Theravada Buddhist monk. Currently Brahm is the Abbot of Bodhinyana Monastery, in Serpentine, Western Australia, the Spiritual Director of the Buddhist Society of Western Australia, Spiritual Adviser to the Buddhist Society of Victoria, Spiritual Adviser to the Buddhist Society of South Australia, Spiritual Patron of the Buddhist Fellowship in Singapore, and Spiritual Patron of the Bodhikusuma Centre in Sydney.

Peter Betts came from a working-class background and went to Latymer Upper School. He won a scholarship to study Theoretical physics at Cambridge University in the late 1960s. After graduating from Cambridge he taught in high school for one year before traveling to Thailand to become a monk and train with the Ajahn Chah Bodhinyana Mahathera. Brahm was ordained in Bangkok at the age of twenty-three by the Abbot of Wat Saket. He subsequently spent nine years studying and training in the forest meditation tradition under Ajahn Chah.

Venerable Ajahn Chah Subhaddo (Chao Khun Bodhinyana Thera) (Thai: ชา สุภัทโท, alternatively spelled Achaan Chah, occasionally with honorific titles Luang Por and Phra; 17 June 1918 – 16 January 1992) was an influential teacher of the Buddhadhamma and a founder of two major monasteries in the Thai Forest Tradition.

Respected and loved in his own country as a man of great wisdom, he was also instrumental in establishing Theravada Buddhism in the West. Beginning in 1979 with the founding of Cittaviveka (commonly known as Chithurst Buddhist Monastery) in the United Kingdom, the Thai Forest Tradition of Ajahn Chah has spread throughout Europe, the United States and the British Commonwealth. The dhamma talks of Ajahn Chah have been recorded, transcribed and translated into several languages.

More than one million people, including the Thai royal family, attended Ajahn Chah's funeral in 1992. He left behind a legacy of dhamma talks, students, and monasteries.

Ajahn Chah was born on 17 June 1918 near Ubon Ratchathani in the Isan region of northeast Thailand. His family were subsistence farmers. As is traditional, Ajahn Chah entered the monastery as a novice at the age of nine, where, during a three-year stay, he learned to read and write. He left the monastery to help his family on the farm, but later returned to monastic life on 16 April 1939, seeking ordination as a Theravadan monk (or bhikku). According to the book Food for the Heart: The Collected Writings of Ajahn Chah, he chose to leave the settled monastic life in 1946 and became a wandering ascetic after the death of his father. He walked across Thailand, taking teachings at various monasteries. Among his teachers at this time was Ajahn Mun, a renowned meditation master in the Forest Tradition. Ajahn Chah lived in caves and forests while learning from the meditation monks of the Forest Tradition. A website devoted to Ajahn Chah describes this period of his life: