Sen Soshitsu XV (??????) refers to the 15th generation head (iemoto) of Urasenke, one of the most widely known schools of Japanese tea.
Hounsai (???) was the religious appellation he received in 1959 from his Zen mentor, and which is the name that he generally is known by to distinguish him as the 15th Sen Soshitsu.
His son succeeded him as the 16th generation head of Urasenke, and inherited the name Soshitsu, in December 2002.
With that, Sen Soshitsu XV discontinued his own use of the name Soshitsu.
He adopted the name Genshitsu (??) to use instead, and thus has officially gone by the name Sen Genshitsu since then.