Tachyon

A tachyon is a hypothetical particle that can travel faster than the speed of light. 

Most scientists do not believe that tachyons exist. Einstein's theory of special relativity states that nothing can accelerate faster than the speed of light, while tachyons are theorized to be constantly traveling faster than the speed of light. If a tachyon did exist, it would have an imaginary number as its mass.

History
E. C. G. Sudarshan, V.K Deshpande and Baidyanath Misra were the first to propose the existence of particles faster than light and named them "meta-particles". After that the possibility of particles moving faster than light was also proposed by Robert Ehrlich and Arnold Sommerfeld, independently of each other. Lately, Similar concept was coined with the term "tachyon" by Gerald Feinberg in  1967.Feinberg G. 1967. Possibility of faster-than-light particles. Physical Review 159 (5): 1089–1105. . See also Feinberg's later paper: Phys. Rev. D 17, 1651 (1978)

In September 2011, it was reported that a tau neutrino had traveled faster than the speed of light in a major release by CERN; however, later updates from CERN on the OPERA project indicate that the faster-than-light readings were resultant from "a faulty element of the experiment's fibre optic timing system".

Other meaning: imaginary mass field
In modern physics the term "tachyon" often refers to imaginary mass fields rather than to faster-than-light particles.Lisa Randall, Warped Passages: Unraveling the Mysteries of the Universe's Hidden Dimensions, p.286: "People initially thought of tachyons as particles traveling faster than the speed of light...But we now know that a tachyon indicates an instability in a theory that contains it. Regrettably, for science fiction fans, tachyons are not real physical particles that appear in nature."A. Sen 2002. Rolling tachyon. Journal of High Energy Physics.  0204 (2002) 048 The idea of those mass fields, comes in part from Quantum field theory, and Classical field theory.

Imaginary mass fields fields have come to play an important role in modern physics.

 References 

Category:Subatomic particles