The term 'INTROSPECTIVE SECT' can be solved by the three-letter word 'ZEN'. ZEN is a school of Mahayana Buddhism that originated in China during the 6th century CE. It emphasizes the practice of meditation and insight into one's true nature, and is known for its emphasis on introspection and self-reflection. The term 'introspective sect' is often used to describe ZEN Buddhism, as it encourages practitioners to look inward and reflect on their own thoughts and feelings.