Oscillator (from the Lat. "oscillo" = "I oscillate") is a physical system that oscillates. The term "oscillator" is used for any system, if the values, which describe it, vary in time. The concept of an oscillator plays an important role in the solid-state theory, the theory of electromagnetic emission, and the theory of vibrational spectra of molecules. Examples of simplest oscillators are a pendulum and an oscillating contour.