It's an object that's designed to be hurled or shot forward, like a bullet or a rocket
In the case of a bullet, spin increases accuracy, ensuring that the 
trajectory will... [DOC] [TLE] Projectile - definition of projectile by The Free Dictionary [PAR] A fired, thrown, or otherwise propelled object, such as a bullet, having no ... 5. 
capable of being thrust or flung forward, as a missile or the tongue of a frog.