This is currently defined in dictionaries as a yet undiscovered particle that has the properties of traveling beyond the speed of light. Physicists will eventually identify their existence.
To understand the concept of tachyons requires the understanding of how a cell emerges and dies. The transformation of a cell into our time-space system requires a slowing down of faster than light activity. This is revealed in a disturbance at the location where such a cell emerges. The reverse occurs when a cell dies.
Certain UFO's are believed to use a form of propulsion similar to the properties of tachyons to propel them over vast distances and planes of existence.