Luminite is a Hardmode ore that drops exclusively from the Moon Lord, usually in stacks of 70-90 (90-110 in Expert Mode). It can be used to craft Luminite Bars, which can then be used to make a majority of end-game items, such as the Solar,Nebula, Vortex, and Stardust tools and armor.
Crafting all items requires 1088 Luminite. This means defeating the Moon Lord 13 - 16 times in normal mode, or 10 - 13 times in Expert Mode.