A subatomic particle whose emission Data uses to pass a brief message into the next cycle of the temporal loop that keeps destroying the Enterprise-D.
Dekyons feature in the Next Generation episode Cause and Effect. The Enterprise is caught in a repeating fragment of time near the Typhon Expanse, each cycle ending in a collision with another starship and the loss of the ship.
The crew piece together what is happening from echoes of earlier loops, and La Forge finds a dekyon field distortion left by them. He suggests that a dekyon emission could carry a very short signal across the reset, and Data modifies his own systems to send one.
In the last cycle the signal surfaces as the number three, turning up in a poker game and on displays around the ship. Data realises that it points to Riker's three rank pips and follows the first officer's plan, saving the ship and the long-lost USS Bozeman.