Icarons were artificially created tachyon particles found in hyperspace. They were named after Icarus. The particles were generated by the collision of blumons and zeccons.
They were useful in constructing hyperspace-capable vessels, like the experimental Hith craft Skel'Ske, as icarons "preferred" to be in hyperspace; there was a much smaller energy demand placed on a ship when it entered hyperspace and took no energy to remain there.
Most societies banned the development of icarons because they caused paranoia and psychotic behavior in carbon-based lifeforms. They were prohibited by the Fourteenth Resolution of the Armageddon Convention. Individuals who underwent a body bepple were particularly at risk. (PROSE: Original Sin)
Another variety, strange icarons, had no harmful effects on organic lifeforms. However, they did interfere with starship jump engines. If a ship entered or exited subspace in an area flooded with strange icarons, their engine would explode. When the Daleks invaded the solar system in 2157, they set up a strange icaron generator on Mercury to prevent aid from arriving from the Earth's colonies. (PROSE: GodEngine)