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Randori (乱取り) is a term used in Japanese martial arts to describe free-style practice. The term literally means "chaos taking" or "grasping freedom," implying a freedom from the structured practice of kata. Randori may be contrasted with kata, as two potentially complementary types of training.
The exact meaning of randori depends on the martial art it is used in. In judo, jujitsu and Shodokan Aikido, among others, it most often refers to one-on-one sparring where partners attempt to resist and counter each other's techniques. In other styles of aikido, in particular Aikikai, it refers to a form of practice in which a designated aikidoka defends against multiple attackers in quick succession without knowing how they will attack or in what order.
In Japan 
The term is used by Aikido, Judo, and Brazilian Jiu-Jitsu dojos outside Japan. In Japan, this form of practice is called taninzu-gake (多人数掛け), which literally means multiple attackers.
In Judo 
The term is described by Jigoro Kano, the founder of Judo, in a speech at the 1932 Los Angeles Olympic Games: "Randori, meaning "free exercise", is practiced under conditions of actual contest. It includes throwing, choking, holding the opponent down, and bending or twisting his arms or legs. The two combatants may use whatever methods they like provided they do not hurt each other and obey the rules of Judo concerning etiquette, which are essential to its proper working." 
In Tenshin Aikido 
In Steven Seagal's Tenshin Aikido Federation (affiliated with the Aikikai), the randori is different from that of Aikikai—the attackers can do anything to the defender (e.g. punch, grab, kick, etc.),
In Kendo 
In kendo, jigeiko means "friendly" free combat as in competition, but not counting the points.
In Karate 
Although in karate the word kumite is usually reserved for sparring, some schools also employ the term randori with regard to "mock-combat" in which both karateka move very fast, parrying and attempting acts of extreme violence with all four limbs (including knees, elbows, etc.) yet only ever making the slightest contact. Total control of the body is necessary and therefore only the senior grades can typically practice randori. In these schools, the distinction between randori and kumite is that in randori, the action is uninterrupted when a successful technique is applied.
In ninjutsu 
Randori is also practiced in Bujinkan ninjutsu and usually represented to the practitioner when he reaches the "Shodan" level. In ninjutsu, randori puts the practitioner in a position where he is armed/unarmed and being attacked by multiple attackers.
See also 
- Original text of this speech available at The Judo Information Site at http://judoinfo.com/kano1.htm