Cai Lun, formerly romanized as Ts'ai Lun, was a Chinese eunuch court official of the Eastern Han dynasty. He occupies a pivotal place in the history of paper due to his addition of pulp via tree bark and hemp ends which resulted in the large-scale manufacture and worldwide spread of paper. Although traditionally regarded as the inventor of paper, earlier forms of paper have existed since the 3rd century BCE, so Cai's contributions are limited to innovation, rather than invention.

A eunuch is a man who has been castrated. Throughout history, castration often served a specific social function. In China, castration included removal of the penis as well as the testicles. Both organs were cut off with a knife at the same time.