Ajahn is the Thai form of the word for teacher, and it is the ordinary respectful title for a senior monk, most often one of at least ten years standing.
The teacher-monk
Derived from the Pali term for teacher, the title is given to a monk of seniority, conventionally one who has passed at least ten rains retreats since ordination and so is qualified to be a preceptor and a teacher of others, so the word marks both seniority in the monastic life and the role of instructing junior monks and lay followers. It is a term of respect and of function rather than a formal rank conferred by an institution, and it attaches to the monk standing and teaching role within the community.
The forest teachers
The title is especially associated in the wider world with the masters of the Thai forest tradition, the lineage of meditation monks who emphasise strict discipline, meditation and life in the forest, many of whom, known as ajahn, became influential teachers whose instruction has spread well beyond Thailand, including to Western monastics and lay practitioners. Entry to the standing is simply by seniority and recognised competence in the monastic life, so the office rests on the accumulated years and the acknowledged role of teacher. The ajahn thus represents the senior teaching monk of Thai Buddhism, and the honorific, like the corresponding titles in other Buddhist countries, marks the passage from junior monk to one qualified to guide and instruct others in the path.