A muzzleloader is any firearm into which the projectile and the propellant charge is loaded from the muzzle of the gun (i.e., from the forward, open end of the gun's barrel). This is distinct from the modern (higher tech and harder to make) designs of breech-loading firearms. The term "muzzleloader" applies to both rifled and smoothbore type muzzleloaders, and may also refer to the marksman who specializes in the shooting of such firearms. The firing methods, paraphernalia and mechanism further divide both categories as do caliber (from cannons to small-caliber palm guns).

Modern muzzleloading firearms range from reproductions of sidelock, flintlock and percussion long guns, to in-line rifles that use modern inventions such as a closed breech, sealed primer and fast rifling to allow for considerable accuracy at long ranges.

Modern mortars use a shell with the propelling charge and primer attached at the base. Unlike older muzzleloading mortars, which were loaded the same way as muzzleloading cannon, the modern mortar is fired by dropping the shell down the barrel where a pin fires the primer, igniting the main propelling charge. Both the modern mortar and the older mortar were used for high angle fire. However, the fact that the mortar is not loaded in separate steps may make its definition as a muzzleloader a matter of opinion.

Muzzleloading can apply to anything from cannons to pistols but in modern parlance the term most commonly applies to black powder small arms. It usually, but not always, involves the use of a loose propellant (i.e., gunpowder) and projectile, as well as a separate method of ignition or priming.