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.

Ajahn Sucitto (Bhikkhu Sucitto, born 4 November 1949) is a British-born Theravada Buddhist monk ("ajahn" is the Thai rendition of ācariya [Pali] "teacher"). He is, since 1992, the abbot of Cittaviveka, Chithurst Buddhist Monastery. He was born in London, and was ordained in Thailand in March 1976. He moved to Britain in 1978 and took up training under Ajahn Sumedho at the Hampstead Buddhist Vihara. In 1979 he was one of the small group of monks, led by Ajahn Sumedho, who established Cittaviveka, Chithurst Buddhist Monastery, in West Sussex. In 1981 he was sent up to Northumberland to set up a small monastery in Harnham, which subsequently became Aruna Ratanagiri. In 1984 he accompanied Ajahn Sumedho in establishing Amaravati Buddhist Monastery in Hertfordshire. In 1992 he was appointed abbot of Cittaviveka.

Ajahn Sucitto's main work has been in teaching, editing and writing, although he was also largely responsible for the creation of the protocols and standards that flesh out the ten-precept training of the sīladharā (nuns) Order.

Ajahn Amaro (born 1956) is a Theravadin teacher and abbot of the Amaravati Buddhist Monastery at the eastern end of the Chiltern Hills in south east England. The centre, in practice as much for ordinary people as for monastics, is inspired by the Thai forest tradition and the teachings of the late Ajahn Chah. Its chief priorities are the practice and teaching of Buddhist ethics, together with traditional concentration and insight meditation techniques, as an effective way of dissolving stress.

Ajahn Amaro was born Jeremy Charles Julian Horner in Kent, England. He was educated at Sutton Valence School and Bedford College, University of London. Ajahn means teacher. He is a second cousin of I.B. Horner (1896-1981), late President of the Pali Text Society.

Apart from a certain interest in the theories of Rudolf Steiner—to which he had been introduced by Trevor Ravenscroft, later perhaps best known as author of the 1970s cult volume The Spear of Destiny (after which a post-punk band named itself)—Amaro's principal enthusiasms on leaving university were, by his own admission, pretty much those standard-issue among sceptical students of the day: sex, drugs and rock'n'roll.