Plasma frequency is the frequency of the oscillation of electron charge density in plasma. The term can also be used for other charged particles, but as electrons are often the lightest by far, they oscillate more readily.
Electrons among ionized atoms tend toward a frequency stemming from relatively heavy positive particles, but a plasma can also include positrons.
Radio waves are strongly attenuated when passing through plasma at below the plasma frequency. For the ionosphere, this is about 1-10MHz, affecting Earth-based radio astronomy. For the interplanetary medium, the plasma frequency is about 9 kHz and for the interstellar medium, about 1 kHz.