periodic motion between two states or extremes
characterized by frequency, amplitude, period, and phase
simple harmonic oscillator: restoring force proportional to displacement (spring, pendulum)
damped oscillation: amplitude decreases over time due to friction or resistance
driven oscillation: external force sustains the motion; resonance occurs when driving frequency matches natural frequency
mechanical: pendulum, vibrating string, seismic waves
electromagnetic: radio waves, light, alternating current
biological: heartbeat, circadian rhythms, neuronal firing, predator-prey cycle
economic: business cycles, inventory cycles, boom-bust dynamics