A dekyon is a subatomic particle which can be both naturally-occurring and artificially generated. In 2368, distortions in the natural dekyon field were left over as "echos" from previous iterations of a temporal causality loop in which the USS Enterprise-D had become trapped. The crew was able to use a dekyon emission to send a message into the next iteration, which set up resonances in Data's positronic subprocessors. The information transmitted allowed Data to avert the disaster which initiated the temporal loop in the first place, thereby freeing the Enterprise. (TNG: "Cause and Effect" )