A musket was an antiquated form of projectile weapon developed and used on Earth in the 16th through 19th centuries. It was loaded with gunpowder and a lead ball, the latter of which was the projectile fired. They may also be equipped with a bayonet attached at the tip of the gun's muzzle.
In 2364, the crew of the USS Enterprise-D were whisked away by Q to an unknown planetoid where they discovered aliens wielding muskets and dressed in 19th century French Army uniforms. As part of Q's fantasy scenario, however, the muskets actually fired phaser beams. Both Worf and Wesley Crusher were killed after being pierced with a musket's bayonet, but were revived by William T. Riker, who had been temporarily granted Q's abilities. (TNG: "Hide and Q")