Hello!

I think you may be thinking too hard about this!

Projectile motion is simply the motion of "a projectile,"

something that is launched, travelling in a curved path. 

Think about the classic example of a cannonball being launched from a cannon;

it will initially travel upwards in a diagonal direction, then travel downwards due to gravitational pull,

eventually forming a trajectory that resembles a rainbow, a curved path. 

The reason why we talk about projectile motion in physics is because

we are developing the skills to identify horizontal force/movement and vertical force/movement 

and that they can each be calculated completely separately 

even though we are talking about the movement of a single object. 

Hope this helps! :)