Theravāda is the oldest Buddhist school, known for its emphasis on monastic discipline. This discipline is based on the teachings of the Buddha and is an essential component of Theravāda Buddhism. The main principles of monastic discipline in Theravāda Buddhism can be summarized as follows:

1. Vinaya: The Vinaya is the code of conduct for monastic life in Theravāda Buddhism. It consists of rules and regulations that govern the behavior and actions of monks and nuns. These rules were established by the Buddha himself and are considered essential for maintaining a harmonious and ethical monastic community.

2. Pātimokkha: The Pātimokkha is a set of 227 rules that are recited by monks and nuns during their fortnightly recitation ceremony. These rules cover various aspects of monastic life, such as proper conduct, ethical behavior, and the use of material possessions. The recitation of the Pātimokkha serves as a reminder for monks and nuns to uphold the Vinaya and maintain their monastic discipline.

3. Bhikkhu/Bhikkhunī: In Theravāda Buddhism, the term "bhikkhu" refers to a fully ordained monk, and "bhikkhunī" refers to a fully ordained nun. These titles are given to those who have taken the vows and committed themselves to the monastic life. The bhikkhu/bhikkhunī is expected to follow the Vinaya and Pātimokkha and live a simple and disciplined life.

4. Renunciation: Renunciation is a fundamental principle in Theravāda Buddhism. Monks and nuns are expected to renounce worldly possessions and attachments and lead a simple and austere life. This includes giving up material possessions, living a celibate life, and abstaining from worldly pleasures.

5. Meditation: Meditation is an integral part of monastic discipline in Theravāda Buddhism. Monks and nuns are expected to spend a significant amount of time in meditation, as it helps them develop mindfulness, concentration, and wisdom. This practice also helps them overcome their desires and attachments and attain enlightenment.

6. Sangha: The Sangha refers to the monastic community in Theravāda Buddhism. It is considered an essential support system for monks and nuns, as it provides them with a sense of belonging and guidance. The Sangha also plays a crucial role in upholding the Vinaya and maintaining monastic discipline.

In conclusion, the main principles of monastic discipline in Theravāda Buddhism revolve around following the Vinaya, Pātimokkha, and other rules and regulations established by the Buddha. Monks and nuns are expected to lead a simple and disciplined life, renounce worldly possessions, and cultivate mindfulness and wisdom through meditation. The Sangha also plays a crucial role in supporting and upholding monastic discipline.