What is Thermionic Emission ?

Thermionic Emission refers to the emission of electrons in the presence of electric potential and temperature.  Thermionic emission occurs when a negative charged electrode is heated.  The heat energy agitates the atoms and the electrons leave the surface of the negatively charged material.
A Vacuum Tube

The rate of emission increases with the temperature.  Thermionic current occurs if an anode which is positive with respect to the cathode is placed near the cathode.