A collective oscillation that occurs in a plasma. To ensure that the plasma is electrically neutral there has to be a positive background to the free electrons. This causes a charge disturbance in the electron density, which the electrons move to screen. The electrons tend to overshoot in doing so and are pulled back, overshoot again, etc., with the result that the charge density is described by a simple harmonic motion, which has a frequency called the Langmuir frequency, named after the US chemist Irving Langmuir (1881–1957), who analysed plasma oscillations in 1928. In plasmas described by quantum mechanics, such as the electron gas and the electrons in a metal plasma, oscillations become plasmons. See also Landau damping.