Newton's first law
Also called Law of inertia
Newton's first law states that if the net force on a system is zero, the velocity of that system stays constant. Rest is only the special case where that constant is zero; moving in a straight line at steady speed obeys the law just as well.
AP Physics 1 states it in EK 2.4.A.3: if the net force exerted on a system is zero, the velocity of that system will remain constant. The condition is zero net force, the vector sum of every force on the system (EK 2.4.A.1), and not an absence of forces. A book resting on a table has two forces on it and satisfies the law exactly.
Constant velocity, not rest. A puck sliding at 4 m/s across frictionless ice with no net force keeps sliding at 4 m/s indefinitely. The everyday sense that moving things stop unless pushed is a statement about friction, not about the law.
The force-balance version has a name. EK 2.4.A.2 calls a configuration of forces with zero net force translational equilibrium, and EK 2.4.A.4 adds the per-axis caveat students skip: forces can balance along one axis and not the other, and the velocity then changes only along the unbalanced direction. A projectile has balanced horizontal forces and unbalanced vertical ones throughout its flight.
The law also does definitional work. EK 2.4.A.5 defines an inertial reference frame as one from which an observer would verify Newton's first law, so the law is what picks out the frames where the rest of mechanics holds.
The property behind it is inertia. Topic 2.4 sets the AP framing, and the net force guide does the summing.