Tai Chi (pronounced, and often spelled as Taiji), is fundamentally a martial art which originated in China. In current times, however, the emphasis is placed on Tai Chi's outstanding health benefits, including mental health and overall well-being.
The slow, rhythmic movements in the Tai Chi form that many are familiar with promote: concentration, mindfulness, tranquility, blood flow, joint rotation, flexibility, strength, and balance. What makes Tai Chi unique, is that benefits listed (and there are many more) are based in the use and flow of chi gong (energy flow and cultivation) and nei gong (internal power exercises).
Tai Chi can be as strenuous or as gentle as one wishes, depending on age, physical conditions, and preference. It can be practiced almost anywhere, in everyday clothes, and without any equipment!