Origin 500 BCE to 400 BCE
A fully ordained Buddhist nun, and the most sharply contested ordination question in Buddhism.
The order was founded in the Buddha's lifetime, at the request of his aunt and foster mother Mahapajapati Gotami and after an initial refusal, and admitted on the condition of the eight garudhammas, rules which subordinate the bhikkhuni order to the bhikkhu order in specific ways, including that a nun of a hundred years' standing must rise to a monk ordained that day. Whether those rules are original or a later insertion is argued by scholars and by nuns, and the argument is not merely academic, since they are the basis of much current practice. A bhikkhuni keeps 311 rules, more than a monk.
The Theravada bhikkhuni lineage died out in Sri Lanka in the eleventh century and in Burma around the same period. Because valid ordination requires a quorum of existing bhikkhunis alongside bhikkhus, the conventional position is that once the line is broken it cannot be restarted, which is why women in Theravada countries have lived for centuries as maechi in Thailand or thilashin in Myanmar, keeping eight or ten precepts, without full ordination and without the legal standing, support or educational access that come with it.
Revival began in 1996 with ordinations at Sarnath and continued at Bodh Gaya in 1998, using bhikkhunis from the East Asian Dharmaguptaka lineage, which never broke, together with Theravada bhikkhus. Several thousand women have now been ordained on that basis in Sri Lanka, where the revival has substantial support. The Thai and Burmese sangha hierarchies do not accept it: the Thai supreme patriarch's 1928 order forbidding monks to ordain women has never been rescinded, and Thai women seeking ordination have generally gone to Sri Lanka. In 2009 the Australian monk Ajahn Brahm participated in a bhikkhuni ordination in Perth and his monastery was expelled from the Thai forest lineage of Wat Pah Pong for it.
The objections are procedural, that the Dharmaguptaka lineage cannot supply a valid Theravada quorum, and the answers are equally procedural, and behind both is a question about authority that no one body in Theravada Buddhism is positioned to settle, since there is no central authority to settle it. Tibetan Buddhism has faced the same problem; the Dalai Lama has repeatedly supported full ordination for women and has said the decision requires collective agreement among Vinaya holders, which has not been reached.