Speedster (fiction)
A speedster is a fictional character in superhero fiction, particularly comic books, whose primary power is the superhuman ability to perform physical and/or mental acts at impossibly high speeds. The most recognizable such superhero is the Flash. The Golden Age Flash resembled the Greek god Hermes or the Roman god Mercury with his winged cap and winged sandals. Another notable speedster is Quicksilver who has been on The Avengers roster, and has had conflicts with the X-Men. Quicksilver isn't connected to a Speed Force, like members of The Flash family.
Plausibility and artistic license
The use of speedsters in fiction requires artistic license due to the laws of physics that would prohibit such abilities. Moving at the speed of sound, for example, would create sonic booms that are usually not heard in such stories. An enormous amount of energy would be required to achieve such speeds, and as some speedsters can actually move close to or at the speed of light, this would cause them to gain near-infinite mass, according to the laws of relativity.