What does "natural frequency" refer to?

Natural frequency specifically refers to the frequency at which a system tends to oscillate when it is not subjected to any external forces or damping. This concept is fundamental in the study of oscillatory systems such as pendulums, springs, and other mechanical systems. Each system has its own unique natural frequency determined by its physical properties, such as mass and stiffness.

When a system is disturbed from its equilibrium position and then released, it will oscillate at its natural frequency until other forces act upon it (like damping due to friction or air resistance). Understanding natural frequency is vital in many fields such as engineering, architecture, and physics, as it helps predict how structures or systems will respond to various stimuli.

The other choices suggest various aspects of oscillation but do not capture the essence of the term "natural frequency." For instance, the idea of sound in a vacuum does not apply since sound requires a medium for propagation. The highest frequency a system can achieve and a frequency resulting from forced oscillation relate to different concepts in wave mechanics that do not define the inherent characteristics of the system itself when it oscillates freely.