Impulse
Also called Impulse of a force
Impulse is the product of the average force exerted on a system and the time interval during which that force acts. It is a vector pointing along the net force, measured in newton seconds, and it equals the area under a force versus time graph.
Two numbers describe a push: how hard, and for how long. Impulse is the pair combined into one quantity.
EK 4.2.A.2 gives that definition and the AP Physics 1 sheet prints it. Units are newton seconds, , which are identical to : impulse and momentum share a unit because they are the same kind of quantity.
The graph definition is the one exams lean on. EK 4.2.A.4 says the impulse delivered to a system by a net external force equals the area under the curve of a graph of that force against time. When the force is not constant, which is the realistic case during a collision, that area is the only honest route to the impulse. Area below the time axis counts as negative.
Direction matters. EK 4.2.A.3 makes impulse a vector with the same direction as the net force exerted on the system, so a force that reverses partway through an interval delivers two impulses that partly cancel.
What an impulse does to the system it is delivered to, and how to use it to find a final velocity, is the impulse-momentum theorem. That guide owns the procedure and the worked cases: impulse-momentum theorem.