What does the term "projectile" refer to in the context of ballistics?

In the context of ballistics, the term "projectile" specifically refers to an object that is propelled through the air as a result of being fired from a firearm or artillery piece. This definition encompasses a wide range of items, including bullets, shells, and rockets. When discussing projectiles, the focus is on the dynamics of their motion once they have been launched, which includes factors like velocity, trajectory, and external forces such as air resistance.

The other choices do not accurately capture the essence of what a projectile is in ballistics. For instance, a substance used in ammunition refers to the materials, such as gunpowder or propellants, that help to launch the projectile but are not the projectile itself. A type of explosive charge describes explosive materials used separately to create a blast but does not define the projectile which is typically the item aimed and fired. Lastly, a device used to measure speed pertains to measuring the velocity of moving objects, which is unrelated to the definition of a projectile itself, as it does not involve the object being propelled. Thus, option C effectively encapsulates the fundamental understanding of projectiles in the field of ballistics.