Resonance
Also called Resonant frequency
Resonance is the large-amplitude response an oscillating system gives when an external force drives it at its natural frequency, the frequency at which the system oscillates when displaced from equilibrium and left alone.
AP Physics C: Mechanics states it directly, in Topic 7.3. EK 7.3.A.4: in the presence of a sinusoidal external force, a system may exhibit resonance. EK 7.3.A.4.i: resonance occurs when an external force is exerted at the natural frequency of an oscillating system. EK 7.3.A.4.ii: resonance increases the amplitude of oscillating motion. EK 7.3.A.4.iii defines the natural frequency as the frequency at which the system will oscillate when it is displaced from its equilibrium position.
What AP Physics 2 does with it. The word resonance does not appear in the AP Physics 2 required course content. What that course tests instead is the boundary-condition version of the same idea: EK 14.6.B.1.ii says the possible wavelengths of a standing wave are determined by the size and boundary conditions of the region to which it is confined, and 14.6.B.1.iii names pipes with open or closed ends and strings with fixed or loose ends. Those permitted frequencies are exactly the ones a string or an air column responds strongly to, and the lowest of them is the fundamental frequency.
What resonance is not. It is not energy from nowhere. The driver keeps doing work every cycle; the amplitude grows because that work arrives in step with the motion instead of fighting it. Damping is what stops the amplitude growing without limit.