From the Wikipedia page 
Sen Sōshitsu (千宗室?) is the name of the head, or iemoto, of the Urasenke school of Japanese tea ceremony. Sen is the family name and Sōshitsu is the hereditary name assumed by the successor upon becoming iemoto. The first person in this line of the Sen family to use the name Sōshitsu was the youngest son of Sen Sōtan; in other words, a great-grandson of Sen Rikyū. He is generally known as Sensō Sōshitsu (仙叟 宗室）, without mention of the family name, and is counted as the fourth generation in the Urasenke family line. The current head of Urasenke is the sixteenth generation, Sen Sōshitsu XVI, who is distinguished by his religious name, Zabōsai.