The hypothetical particle that travels faster than light is called a tachyon. The name comes from the Greek word tachys, meaning “swift,” and describes a proposed particle that would always move faster than light rather than being accelerated across the light-speed barrier.
Physicists have discussed faster-than-light particles since before and during the development of relativity, but Gerald Feinberg coined the term tachyon in a 1967 paper. In the original idea, tachyons were associated with imaginary mass and unusual energy behavior: as their energy decreased, their speed would increase. Infinite energy would be required to slow one down to the speed of light.
No tachyon has ever been detected, and modern physics treats the concept as speculative. If tachyons could carry information, different observers could disagree about the order of events, potentially allowing messages into the past and creating causality paradoxes. This is one reason they are viewed skeptically.
A common mix-up is the neutrino. Neutrinos are real particles, but experiments have not shown them to travel faster than light. “Tachyonic field” is also a related but different term for an unstable quantum field, not confirmed superluminal particles.