In Zen Buddhism, a superficially nonsensical
question or riddle used by a Zen master to
help a pupil achieve satori (enlightenment).
It is used in the Rinzai school of Zen. A
koan supposedly cannot be understood through
the processes of logic; its solution requires
attainment of a higher level of insight. An
often repeated example is `What is the sound
of one hand clapping?' An answer would be
that the word `clapping' by definition
involves two hands, and therefore this
particular koan is unanswerable.