Final answer:
The natural frequency of a system is the frequency at which it will oscillate without external forces or damping. When a periodic force is applied at this frequency, resonance occurs.
Step-by-step explanation:
The natural frequency of a system refers to the frequency at which the system will oscillate if not affected by driving or damping forces. It is the frequency at which a system undergoes free vibration without external forces or damping. When a periodic force is applied to a system at its natural frequency, it produces resonance, and the system is said to resonate.