Cledonism is a an ancient system of divination based on the presage of good or evil when certain words were spoken without premeditation whenever individuals happened to meet, The system, also, prescribed words to be used on particular occasions.

Cicero said the Pythagoreans were very attentive to these presages, and according to Pausanus, the system was a favorite divination at Smyrna, where the oracles of Apollo were interpreted. A.G.H.

Source: 9.