Ballistics is the science of mechanics that deals with the launching, flight, behavior, and effects of projectiles, especially bullets, gravity bombs, rockets, or the like; the science or art of designing and accelerating projectiles so as to achieve a desired performance.
A ballistic body is a body with momentum which is free to move, subject to forces, such as the pressure of gases in a gun or a propulsive nozzle, by rifling in a barrel, by gravity, or by air drag.
A ballistic missile is a missile only guided during the relatively brief initial powered phase of flight, whose trajectory is subsequently governed by the laws of classical mechanics, in contrast (for example) to a cruise missile which is aerodynamically guided in powered flight.
Science of ballistics as related to firearms is really divided into three separate but inter-related fields:-
Internal Ballistics – The study of what happens inside of the firearm.
External Ballistics – The study of what happens during the bullet’s flight.
Terminal Ballistics – The study of what happens when the projectile strikes the target.