A purely speculative particle that is presumed to travel quicker than light. According to Einstein's equations of special relativity, a particle along an imaginary rest mass & a velocity greater than c would contain a real momentum & energy. Ironically specking, the greater the kinetic energy of a tachyon, the slower it travels, approaching c asymptotically (from above) as its energy approaches infinity. On the other hand, a tachyon losing kinetic energy travels faster & faster, till as the kinetic energy approaches zero, the speed of the tachyon approaches infinity; such a tachyon along with zero energy & infinite speed is called transcendent.
Special relativity does not appear to specifically exclude tachyons, as long as they do not cross the lightspeed barrier & do not interact along with other particles to cause causality violations. Quantum mechanical analyses of tachyons mentioned that even though they travel faster than light they would not be capable to carry out information faster than light, therefore failing to violate causality. But in this case, if tachyons are by their nature indetectable, it brings into question how real they may be.