Rinzai Zen is a school of Zen Buddhism that emphasizes the use of koans, or paradoxical riddles, to provoke enlightenment experiences in its practitioners. It was founded by the Chinese Zen master Linji Yixuan in the 9th century, and was later brought to Japan by Eisai, who established the first Rinzai Zen temple in the country in the 12th century.

The name "Rinzai" comes from the Japanese pronunciation of Linji, and the school is also known as the Linji school in China. It is one of the three main schools of Zen Buddhism, along with Soto and Obaku, and has a strong presence in Japan, Korea, and the West.

Rinzai Zen places great emphasis on the practice of zazen, or seated meditation, as a means of attaining enlightenment. However, unlike other schools of Zen, Rinzai also incorporates the use of koans as a central part of its practice. These koans, which are often nonsensical or paradoxical statements, are meant to disrupt the logical thinking mind and allow for a direct experience of reality.

The practice of koans is typically done in a one-on-one setting between a student and a teacher, known as a dokusan. The student presents their understanding of a koan to the teacher, who then gives feedback and further instructions. This process is repeated until the student has a breakthrough or realization, known as a kensho, which is considered a step towards enlightenment.

Rinzai Zen also places importance on the use of physical discipline, such as rigorous meditation schedules and strict adherence to monastic rules. This is believed to help practitioners develop mental and physical discipline, which is necessary for deepening their understanding of Zen.

Hakuin Ekaku, also known as the "Reviver of Rinzai Zen," played a crucial role in revitalizing and popularizing the school in Japan during the 18th century. He emphasized the use of koans and is credited with creating the famous koan "What is the sound of one hand clapping?" which has become synonymous with Rinzai Zen.

In summary, Rinzai Zen is a school of Zen Buddhism that emphasizes the use of koans and physical discipline as a means of attaining enlightenment. Its focus on direct experience and the use of paradoxical statements sets it apart from other schools of Zen and has made it a popular practice for those seeking a deeper understanding of reality.