Having been a student of tai chi, I'd like to rebut something stated in this article: Tai Chi -- A Cure for Insomnia in Older Adults? that tai chi is not a form of martial arts.
Tell that to other martial arts students, practitioners and my sifu. Sure, the focus upon the combat aspect has lessened over the centuries as more focus has been on the control, discipline, calming movements and engaging your internal chi. But, let me remind you that "tai chi chuan" translates to "grand ultimate fist." Every movement is to either defend or disarm.
There are at least 24 movements. Each movement develops balance, strength, concentration and reflexes through control, fluidity and sequence. Every movement follows a specific order.
Learn tai chi from a proper sifu (master), and you will learn it as more than an exercise, you WILL learn it as a martial art. It should not be relegated to the level of a health and fitness fad.
Tai Chi for Health, Combat and Spiritual Cultivation
Tai Chi for Older People Reduces Falls, May Help Maintain Strength