Well I know what a tachyon is. A tachyon is a hypothetical particle that travels faster than the speed of light in a vacuum. The tachyon is a theoretical construct which was derived from Einstien's relativistic equation governing the mass-increase of objects as they approach light speed. Normally, any particle with mass can only have a solution to the equation which is slower than light.

However, if you work backwards and state the solution as being faster than light, you get a mass value which is 'imaginary', which means that it is a multiple of the square root of -1.

Thus, tachyons would have a mass value which is an imaginary number.

Interestingly, the speed is light is still a barrier to tachyons. Tachyons are predicted to travel faster than the speed of light, but they cannot travel slower than light. This is the exact opposite of normal matter particles.

Tachyons have not been discovered yet, however.