"Tachyon" literally means "speedy particle". It's used to refer to a particle traveling faster than the speed of light. Special relativity relates velocity to e

"Tachyon" literally means "speedy particle". It's used to refer to a particle traveling faster than the speed of light. Special relativity relates velocity to elapsed time for a particle, and the upshot of that is that anything with positive squared mass must be traveling at less than the speed of light. When you learn about quantum field theory you find out that particles can be modeled as linearized modes of fields. e.g. the photon is a mode of the electro-magnetic field. This is a more general theory than just treating particles as fundamental, because it makes predictions for creation and annihilation of particles. Again, squared mass appears in these formulea. If you write down a field theory with a squared mass term that is _negative_, you might naively think that this gives you particles that travel faster than the speed of light. However, the modes of such a field would not oscillate - they would grow exponentially (at least until they get out of the linear regime). This is what is meant by a "tachyonic instability".