Terminal velocity

The AP Physics C: Mechanics CED defines it in EK 2.9.A.3 as the maximum speed achieved by an object moving under a constant force and a resistive force exerted in opposite directions, with the terminal condition reached when the net force on the object is zero.

Zero net force means zero acceleration, so terminal velocity is a Newton's first law state rather than a Newton's second law one. Nothing stops. The object keeps going at constant velocity.

A resistive force is velocity dependent and points opposite the velocity, the CED's worked example being $\vec{F}_r = -k\vec{v}$ in EK 2.9.A.1. For an object falling under that drag law the terminal condition is $mg = kv_t$, giving $v_t = mg/k$. Heavier falls faster: double the mass at the same drag coefficient and the terminal speed doubles.

The approach is asymptotic. Velocity, position and acceleration all come out exponential in time under a resistive force of that form (EK 2.9.A.2.iii), so the object never quite reaches $v_t$, it only closes on it. A sketch of $v$ against $t$ should flatten toward a horizontal asymptote, never kink into a flat line.

This sits in AP Physics C: Mechanics topic 2.9 because getting the velocity function needs separation of variables. AP Physics 1 treats air resistance as negligible unless a question says otherwise, so free fall there has no terminal speed.