A muzzleloader is any firearm

Firearm

A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...

 into which the projectile

Bullet

A bullet is a projectile propelled by a firearm, sling, or air gun. Bullets do not normally contain explosives, but damage the intended target by impact and penetration...

 and usually the propellant

Propellant

A propellant is a material that produces pressurized gas that:* can be directed through a nozzle, thereby producing thrust ;...

 charge is loaded from the muzzle of the gun

Gun

A gun is a muzzle or breech-loaded projectile-firing weapon. There are various definitions depending on the nation and branch of service. A "gun" may be distinguished from other firearms in being a crew-served weapon such as a howitzer or mortar, as opposed to a small arm like a rifle or pistol,...

 (i.e. from the forward, open end of the gun's barrel). This is distinct from the more popular modern (higher tech and harder to make) designs of breech-loading firearms. The term "muzzleloader" may also apply to the marksman who specializes in the shooting of ML Firearms. The term of art is not meant to connote anything about whether the weapon's barrel had received the further machining step of rifling

Rifling

Rifling is the process of making helical grooves in the barrel of a gun or firearm, which imparts a spin to a projectile around its long axis...

 the barrel, so there are two broad classifications: rifled muzzleloaders and smooth bore muzzleloaders.