Powder
Also: Propellant
The propellant inside the cartridge. Burns rapidly when ignited by the primer, generating gas that pushes the bullet down the bore.
Modern smokeless powders come in single-base (nitrocellulose) and double-base (nitrocellulose + nitroglycerin) formulations, in stick, ball, and flake forms. Each cartridge has a small list of powders that perform well in it: H4350 for 6.5 Creedmoor, Varget for .308, Retumbo for .338 Lapua.
Storage matters more than handloaders expect. Keep powder in original sealed containers, in a cool dry place, away from heat and ignition sources. Inspect for color change or off-smells before loading from a container that’s been around a few years.