A tachyon is a hypothetical particle that travels faster than the speed of light. According to the theory of relativity, nothing can exceed this speed, but tachyons are thought to exist in a realm where they can. If they were real, they would have some unusual properties, such as moving backward in time and having imaginary mass. Scientists have not yet found any evidence of tachyons, and they remain a topic of speculation in physics. They are often discussed in relation to concepts like quantum mechanics and string theory, which explore the fundamental nature of the universe and the behavior of particles at incredibly small scales.