Seika (Saika 最: upmost; most; extreme) Tanden [臍下丹田]

The characters/ideograms mean "center of the body; pit of the stomach." The first character means, "navel," the second character means, "rust-colored; red; red lead; pills," the third character means, "rice field; rice paddy." 

Seika is the Japanese characters. Also known as  seika no itten [臍下の一点] (one point below the navel), kikai tanden [気海丹田] (ocean of energy below the navel) or kikai [気海] (ocean of energy).

The importance of the tanden comes from where our center of gravity resides and that speaks to the various principles that are fundamental to martial systems such as karate, aikido and judo, etc. We are taught from the beginning to focus on our tanden, the center of our bodies, that being the lower abdomen. In truth the center resides just below the navel, some say about two inches below, but also that center moves toward the actual center, i.e. that point that resides somewhere below the naval and about mid way between that point and the spinal column at our back. 

This focus and development means the creation of the foundation to power in the body. This is to supplement the use of muscles throughout the body, i.e. the arms, the legs, etc. It is pulled from the tanden and directed outward through the arms and legs, etc. 

As stated in a fictional book but holds truth in its foundation, "If you do not learn to breathe properly, you will learn to do nothing properly." As a basic or fundamental to any martial art, you must practice proper breathing from the seika tanden until it becomes instinctive. By the breathing you trigger chemicals that calm you and calm controls fear and anger. This in turn controls the heart keeping a better control on the pressure and beats per minute. All this to remain in as much control of self as possible and with repetitive training toward reality you allow the training when trained to instinctive action you control the whole so action is appropriate. 

This breathing and development of the seika tanden is paramount to making any martial art work in a stressful dangerous and possibly debilitating encounter. Debilitating not by the action of an adversary but by the reaction of the body-mind of you, the person in defense of self. 

Focus of the mind and the activation of any movement must start from the tanden. In addition, the musculature in association with the organs and the connections to the spine in proper alignment are also key to development of the seika tanden for martial arts. Those bands of abdominal muscles and associated connections should be the focus of development not only in strength but in the other principles such as structure, alignment and posture, etc.

Again, even in this explanation of the Asian fixation on the tanden in martial arts the fundamental principles of martial systems align themselves with the concepts and context of traditional martial arts training and practice regardless of the style/system involved. 

Bibliography:

Shinkitai Karate. Saika tanden: center of the body. 

http://www.shinkitaikarate.ca/Scona/Glossary/S/saika%20tanden/saika%20tanden.html