According to the Physics Classroom, projectile motion occurs when an object that is at rest or something close to rest is projected or dropped. An object under projectile motion is an object upon which the only acting force is gravity.
An object becomes a projectile if it dropped from rest, as long as air resistance is negligible or non-existent, or thrown from rest, assuming air resistance is negligible. Any object that continues in motion from its own inertia and is influenced only by gravity can be considered to be an object under projectile motion. Gravity works on the projectile downward, causing a downward acceleration.