You’re probably thinking of Cherenkov radiation. Nothing has ever been observed moving faster than c (that would be a tachyon), and light travels at c in a vacuum, but that doesn’t mean that nothing travels faster than light. For instance, light travels through water at about .7 c. It’s possible for some other particle (let’s say it’s an electron) to travel through water at .8 c or .9 c. In that case, the electron is travelling through water faster than light travels through water, and you get something called Cherenkov radiation, which is analogous to the sonic boom a plane produces when it travels faster than the speed of sound, or the wake a boat produces when it travels faster than the speed of the surface waves.
The key to understanding Einstein is that he made the speed of light into a constant to simplify his math. If he had been born 50 years later, with computers to deal with the variations of the speed of light, the world of physical mathematics would more closely resemble the real world.
There are no vacuums. Therefore the speed of light in a vacuum is not a measure.
Chronos, that’s what I was thinking of. Thanks.