Projectile
Also called Projectile motion
A projectile is an object that has been launched and is then moving under gravity alone, with no engine and no air resistance. AP models its flight as motion with zero acceleration in one dimension and constant nonzero acceleration in the other.
The word describes a condition, not a kind of object. A cannonball, a thrown set of keys and a package dropped from a plane are all projectiles once nothing but gravity acts on them. A rocket with its engine burning is not one.
AP Physics 1 EK 1.5.B.2 and C: Mechanics EK 1.5.A.4 use identical wording: projectile motion is a special case of two-dimensional motion that has zero acceleration in one dimension and constant, nonzero acceleration in the second dimension. With horizontal and vertical axes that reads as constant horizontal velocity and vertical acceleration downward.
The two dimensions do not talk to each other. AP Physics 1 EK 1.5.B.1 lets you analyse two-dimensional motion with one-dimensional kinematic relationships once it is split into components, and C: Mechanics EK 1.5.A.3 gives the reason: motion in one dimension may be changed without causing a change in a perpendicular dimension. A ball rolled off a table and a ball dropped beside it hit the floor together.
Air resistance is assumed away. The AP Physics 1 exam conventions state that it is negligible unless a question says otherwise, which is what keeps the horizontal velocity constant.
Two readings students reverse: at the highest point the vertical velocity is zero while the acceleration is still 9.8 m/s downward, and the horizontal velocity never changes at any point, top included.
The solving routine belongs to how to solve projectile motion problems; check numbers with the projectile motion calculator.