Smokeless Powder
The propellant in essentially every modern centerfire cartridge. Burns clean compared to the black powder it replaced in the late 1800s.
Smokeless is really a bit of a misnomer. It still produces some smoke, just dramatically less than the black powder used through the 19th century. The big improvements: higher energy per grain, no fouling cake building up in the bore between shots, and consistent burn rates that make load development meaningful.
Modern smokeless is nitrocellulose-based (single-base) or nitrocellulose + nitroglycerin (double-base). Burn rate is the variable that matters for cartridge-powder pairing.