You hear it all the time -- that so-and-so is "in the limelight" or "stealing the limelight" -- and while we all know what people mean when they say that, we may not have any idea what limelight actually is. It's one of those phrases that was a lot more literally true when it was coined, back in the 19th century, when theaters actually used limelight to illuminate their stages. Used for the first time in London's Convent Garden Theater in 1837, it had come into widespread use around the world by the 1860s. They were employed as ultra-bright spotlights that shone on the performers center stage, focusing the audience's attention while regular gaslights lit the rest of the theater. By the end of the century, electric arc lights had replaced limelight -- but the phrase lived on.
Limelight is created by exciting the atoms in a chunk of lime (also known as calcium oxide) by applying a flame to it, which creates an intense glow. It has other uses as well, most notably for breaking down organic materials (like corpses) and if you're creative, like King Henry III, for blinding one's enemies during battle. According to historian David Hume of Godscroft, Henry's navy destroyed an invading French fleet by, having "gained the wind ... throwing in their faces a great quantity of quicklime, which he purposely carried on board, he so blinded them, that they were disabled from defending themselves." (Don't try that at home, kids.)
Here's a fun clip showing what happens when you burn lime: