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")
In 2371, the crew of the USS Voyager was able to use a dekyon beam to widen a breach in the event horizon of a type-4 quantum singularity in which the ship had become trapped. As Voyager could only safely approach the breach at a range of fifty million kilometers, due to the fact that the ship's warp engines would cause the breach to collapse, Captain Janeway and Lieutenant B'Elanna Torres piloted a shuttlecraft closer to the breach, and successfully used the dekyon beam to widen the breach enough for Voyager to escape through (VOY: "Parallax")