Seiza is written with two Chinese characters. The character pronounced sei- means "proper, right, true," and by itself can be pronounced tadashii, meaning "just right" or "proper." The character -za means "sitting posture," and is written with an ideograph depicting people sitting on a raised platform under a roof. To the Japanese, sitting in seiza, with one's shins folded under, is the most efficient, beautiful, and "proper" posture while engaged in a formal activity sitting on tatami mats indoors.
Quote taken from: http://www.furyu.com/archives/issue6/Seiza.html
Humorous definition: Seiza is a way of sitting that will guarantee that your legs go to sleep within 15 minutes and you will fall over when you try to stand up.