In Zen Buddhism, a koan (pronounced "KO-ahn") is a short puzzle. The student is meant to contemplate the puzzle and seek its answer through meditation. The most famous koan is probably, "What is the sound of one hand clapping?" Sometimes, the koan is a parable, but always the meaning is elusive and, if discerned, instructive. Parable koans often end with the phrase, "the student was enlightened," signifying that it is necessary to truly understand the koan to understand the Buddha and achieve enlightenment.
In Zendo, a koan is a collection of Icehouse pieces that has certain observable characteristics. These characteristics cause the master to mark the koan with either a black stone or white stone according to the master's secret rule. When a student guesses the rule, that student has acheived enlightenment.
Most Zendo koans have two to five pieces in them, though theoretically they can have any number from one to sixty. Most people do not believe that a koan with zero pieces ("the null koan") is a koan at all. Contemplating this, you may be enlightened.