Tiantai, also known as the Tiantai school of Buddhism, is a Mahayana Buddhist tradition that originated in China during the 6th century. It is named after the mountain where its founder, Zhiyi, established his monastery. Tiantai is a comprehensive and systematic approach to Buddhist teachings, with a focus on the Lotus Sutra as its central text.

The core principle of Tiantai is the integration of all Buddhist teachings around the Lotus Sutra. This means that all other Buddhist teachings, including the Theravada and other Mahayana teachings, are seen as complementary and supportive of the ultimate truth revealed in the Lotus Sutra. This approach is known as "one vehicle" or "one teaching" and is based on the belief that the ultimate truth is universal and can be accessed by all beings.

One of the key teachings of Tiantai is the concept of the "three truths." These truths are the provisional truth, the relative truth, and the ultimate truth. The provisional truth refers to the teachings that are adapted to the understanding and capacity of different individuals, while the relative truth refers to the conventional reality of the world. The ultimate truth, on the other hand, is the ultimate reality and the ultimate goal of all Buddhist teachings.

Tiantai also emphasizes the concept of "three thousand realms in a single moment of life," which refers to the belief that all beings possess the potential to attain enlightenment in this lifetime. This concept is based on the idea that all phenomena are interconnected and that all beings have the Buddha nature within them.

In addition to the Lotus Sutra, Tiantai also places importance on the teachings of the Avatamsaka Sutra, which emphasizes the interdependence and interconnectedness of all phenomena. The practice of Tiantai involves the study and contemplation of these sutras, as well as the practice of meditation and other forms of spiritual cultivation.

One of the unique aspects of Tiantai is its emphasis on the Middle Way, which advocates for a balanced approach to spiritual practice. This means avoiding extremes and finding a harmonious balance between the teachings of emptiness and the teachings of form.

In summary, Tiantai is a comprehensive and inclusive approach to Buddhist teachings that centers around the Lotus Sutra. It emphasizes the interconnectedness of all phenomena and the potential for all beings to attain enlightenment in this lifetime. Its teachings of the three truths and the Middle Way provide a balanced and harmonious path towards ultimate truth and liberation.