Pali, masculine noun, literally means 'elder'.
In Theravada Buddhism, Thera is used as an honorific or title for monks who have seniority in a temple or monastery, or who have distinguished themselves in some way.
There are four ways that a monk might distinguish himself, traditionally. They are:
- Morality (sila)
- Mastery of important teachings of the Buddha
- Skill in meditation
- Attainment of enlightenment, or of some intermediate step towards that goal (such as once returner)
Additionally, in the modern age, the title of Thera or Mahathera
(great elder) is occasionally bestowed on monks who have distinguished themselves by the governments of Theravada
The female equivalent of the term, Theri, is used as the title of the senior most nun in a Theravada nunnery. The masculine title may have originally held the same meaning, but evolved following the extinction of the bhikkhuni order in the late 13th Century A.D.
In both male and female orders, seniority is calculated based on the date of an aspirant's entry into the order, rather than their personal age. This practice persists today in Theravada monasteries- a monk who is ordained only minutes before another will technically be his senior for their entire careers. However, variation in record keeping and the fact that many monks may be ordained at once on an uposatha day or at the start of vassa means that this is at best an approximate method.
As with most attainments in Buddhism, it is considered bad form to adopt the title of 'Thera' for oneself, regardless of age or attainment. Thera is what others call a senior monk out of respect, not what they call themselves. Claiming the title for oneself on the basis of spiritual attainment may even be regarded as a violation of the monastic code.