Tiantai, also known as the Tendai school in Japan, is a Mahayana Buddhist tradition founded by the Chinese monk Zhiyi in the 6th century. One of the key principles of Tiantai is the integration of all Buddhist teachings around the Lotus Sutra, which is seen as the ultimate and most profound teaching of the Buddha. In this context, Tiantai also has a unique perspective on the concept of ultimate reality.

According to Tiantai, ultimate reality is referred to as "Suchness" or "True Suchness" (zhenru in Chinese). It is the ultimate nature of all phenomena, the ultimate truth that underlies all existence. This concept is similar to the Mahayana concept of emptiness, but Tiantai emphasizes that Suchness is not mere emptiness or nothingness. Instead, it is the ultimate reality that encompasses both emptiness and existence, and is the unchanging and eternal nature of all things.

Tiantai also teaches that ultimate reality can be realized through the practice of meditation and contemplation. Zhiyi developed a unique meditation method called the "Fivefold Contemplation" which involves the contemplation of the five aspects of reality - emptiness, provisional existence, middle way, ultimate existence, and the Middle Way. Through this practice, one can penetrate the true nature of reality and achieve enlightenment.

Furthermore, Tiantai views ultimate reality as having two aspects - absolute and relative. The absolute aspect refers to the ultimate nature of all things, while the relative aspect refers to the ever-changing and impermanent nature of phenomena. Tiantai teaches that both aspects are equally important and interdependent, and one cannot exist without the other. This understanding of ultimate reality helps practitioners to see the world as it is, without clinging to fixed concepts or views.

In Tiantai, ultimate reality is also seen as the ultimate goal of Buddhist practice. By realizing the true nature of reality, one can attain liberation from suffering and achieve enlightenment. This is in line with the teachings of the Lotus Sutra, which emphasizes the potential for all beings to become Buddhas and attain ultimate reality.

In conclusion, Tiantai views ultimate reality as the ultimate truth that underlies all existence, and it can be realized through the practice of meditation and contemplation. It is the ultimate goal of Buddhist practice and is seen as the key to achieving enlightenment. Through its unique perspective on ultimate reality, Tiantai offers a profound understanding of the nature of existence and the path to liberation.