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The theory is that it is all encompassing. Ofcourse not all MA is suited for a fight with rules inside an cage. The ones that seem to be most used today are muay thai, BJJ, Boxing/Kickingboxing and Wrestling. Alot of the athletes have backgrounds in stuff like Karate but most don’t directly use it because they are less effective here due to the rules.

There isn’t really a list. It’s kind of all inclusive. The most prominent would be muay thai, jiu-jistu, wrestling, and a few others. But it really does include anything you can think of as practitioners of very strange martial arts can find their way into MMA competitions.

Any MA u want. Hence the word "Mixed" Martial Arts.

Pretty much everything except for Tai Chi. Cardio Kickboxing and Wushu.

since im bored: karate, taekwondo, hapkido, judo, bjj, wrestling, boxing, kickboxing, jujutsu.

There is no definition. Mixed Martial Arts means a mixing of styles. In UFC, the most popular are Muay Thai, Kickboxing, Wrestling, and Brazilian Jujutsu

Pretty much all of them. The most common are Western wrestling (folkstyle, freestyle, Greco-Roman, and Catch/shoot), Muay Thai, Brazilian Jiujitsu, Judo, Western boxing, and Japanese kickboxing, though karate, Taekwondo, Sambo, and even Capoeira, Wing Chun Kung Fu, and Bujinkan Taijutsu have been used. If it teaches you to hit or grab someone, you can use it in MMA.