What is a plasma? Define plasma frequency.

A plasma is a state of matter consisting of highly ionized gas, with roughly equal numbers of positively and negatively charged particles.

Plasma frequency (ω_p) is the natural frequency at which a plasma oscillates when perturbed. It's the frequency at which electrons oscillate collectively against the background of positive ions. It's given approximately by \( \omega_p = \sqrt{\frac{n e^2}{\epsilon_0 m_e}} \), where n is the electron density, e is the electron charge, ε₀ is the permittivity of free space, and m_e is the electron mass.