What is luminosity?

Later this decade, the Large Hadron Collider will be upgraded to the High-Luminosity LHC. What does "luminosity" mean in particle physics? Even on the hottest and driest days, rays from the sun are too weak to ignite a fire. But with a magnifying glass (or, in some unfortunate cases, a glass garden ornament), you can focus sunlight into a beam bright enough to set tinder ablaze. At the Large Hadron Collider, scientists apply this same principle when focusing beams of protons (or sometimes heavy ...