"What is the Lethani?" Serious. Tempi looked at me for a long moment, then suddenly burst out laughing. "I do not know. And I cannot tell you." He laughed again. Understatement. "Still we must speak of it." 
The Lethani is a Zen-like concept originating with the Adem people encapsulating the essence of correct human action. It is best understood by the more intuitive and ego-less aspect of the mind. Kvothe often realizes its intricacies via spinning leaf.
The Adem have several strict customs surrounding the teaching and discussion of the Lethani.
- The Wise Man's Fear, Chapter 87, "The Lethani"