Natural frequency refers to the specific frequency at which a system tends to oscillate when not subjected to any external force. Every physical system, such as a pendulum or a spring, has its own natural frequency determined by its mass and stiffness. When disturbed, these systems will vibrate at this frequency until the energy dissipates. In engineering and physics, understanding natural frequency is crucial for designing structures and machines. If an external force, like wind or an earthquake, matches the natural frequency of a structure, it can lead to resonance, potentially causing significant damage. Thus, engineers must consider natural frequencies to ensure safety and stability.