A kind of Japanese pickled radish, usually yellow in color.
More interesting, however, is who the pickle is named after - a Zen Buddhist monk named Takuan Soho, born in 1573. Besides being a prominent monk, he is most remembered for his role as a spiritual advisor to the young Miyamoto Musashi, who is widely regarded as the greatest Japanese swordsman ever. Takuan gave Musashi his new name after a symbolically killing him by tying Musashi up in a tree for a few days.