A pistol is a small handgun. Unlike a shotgun or a rifle, you can hold and fire a pistol with one hand.
Sometimes the word pistol is used for any handgun, but many people distinguish between the pistol, with its single chamber, and the revolver, which has a cylinder that rotates and holds many bullets. The earliest pistols were used in sixteenth century Europe, and the English word comes from the Middle French pistolet, meaning both "small firearm" and "small dagger." You can also call someone who's full of energy a pistol.