The kata Seisan (十三) (alternate names Sesan, Seishan, Jusanpo, Hangetsu in Shotokan) literally means ’13’. Some people refer to the kata as ’13 Hands’, ’13 Fists’, ’13 Techniques’, ’13 Steps’ or even ’13 killing positions’; however, these names have no historical basis.
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Seisan is thought to be one of the oldest kata, and like many aspects of Okinawan Karate, can trace its roots back to China.
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The kata and its variations can be found throughout the many styles of karate on Okinawa today, such as Goju-ryu, Uechi-ryu, Isshin-ryu, Ryuei-ryu, Seibukan Shorin-ryu and more, also found in some of the Japanese mainland styles such as Shito-ryu, Wado-ryu (Seishan) and Shotokan (Hangetsu).
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There are numerous theories regarding the name of the kata. These include the number of steps originally in the kata, the number of different types of ‘power’ or ‘energy’ in the kata, the number of applications, or that the kata represents defense against 13 specific types of attack. Although none of these explanations have actual historical support.
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While it is believed Seisan derives from Yong Chun White Crane Boxing from Fujian Province in Southern China, where the form is known as ‘Four Gate Hands’, this remains completely unproven and uncorroborated.
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Versions of Seisan taught today have roots in Shuri-te, Naha-te and Tomari-te streams of karate from Okinawa.
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One thing most researchers agree upon is that the movements for Seisan, in all of its original variations, were likely imported from Fuzhou, China. Specifically, it seems likely that it was part of the training regime of the White Crane families who resided there.
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Over time Seisan has grown on Okinawa while fading in China to the point where a direct version of the form can no longer be found in Chinese styles. The variations found on Okinawa contain pieces of the original concepts, stylistically emphasizing different ideas in ways that agree with the overall principle of each Okinawan method.
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The growth and popularity of the kata over time has moved it away from its roots; on the other hand the kata has grown into a distinctly popular and effective Okinawan method. 
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Photo Credit: 沖縄空手世界大会 OKINAWA KARATE WORLD TOURNAMENT Uechi-ryu Seisan.