A harmonic oscillator is a system that experiences a restoring force proportional to its displacement from an equilibrium position. This means that when the system is disturbed, it will oscillate back and forth around that position. Common examples include a mass on a spring or a pendulum. The motion is periodic, meaning it repeats at regular intervals. Harmonic oscillators are characterized by their frequency, which determines how fast they oscillate. The simplest form is the simple harmonic motion, where the motion is sinusoidal. These systems are fundamental in various fields, including physics, engineering, and even quantum mechanics.