Dantien is a concept which frequently arises in Asian philosophy, medicine, and martial arts. The dantien is the body's center of gravity, and in theory the storage center for qi, or life force. It is located just below and behind the navel, although some traditions suggest that the body actually has three points of dantien, at the navel, heart, and forehead. Students in courses which deal with traditional Chinese philosophy such as qi gong classes are often introduced to the concept of dantien at an early stage.
You may also see dantien spelled as dan tien, dantian, tan t'ien, or so forth, due to the fact that Romanization from Chinese characters is often highly imperfect. The term translates as “red field,” and many people visualize the dantien as a physical field of energy, in addition to a spot on the body. Dantien is sometimes thought of as a ball of energy in the body which can be manipulated or adjusted.
Philosophy aside, the stomach is very close to the center of gravity for the human body, and as athletes know, learning to utilize the center of gravity can help to generate more power. Being aware of the center of gravity and bringing it low in the stomach can also improve balance, which is useful for athletes like boxers and fencers, as it can prevent a fall when the body is extended in an attack. The concept of dantien, therefore, is very important in martial arts, with instructors encouraging students to center their energy in the dantien for more powerful blows and greater balance.
There are a variety of contexts in which dantien may come up. In meditation, people may be encouraged to breathe deeply into their dantien, and to focus their minds on the energy it contains. The dantien is also used in Traditional Chinese Medicine, with practitioners exploring the flow of the body's energy and the amount of energy stored at this key point in the body. Some practices such as qi gong are supposed to improve the flow of energy and strengthen the dantien, chambering energy for later use.
The idea of the lower belly as the seat of life force and energy is hardly unique to Asia. This part of the body has potent symbolism in many cultures, and it appears in a wide variety of traditional medicinal practices and sports, although it may not always be explicitly defined as a powerhouse of mental energy in Western athletics.