Of or pertaining to the emission of electrons by heat. Electrons emitted from something due to heating (such as the cathode of a vacuum tube) are known as thermal ions, or thermions. Thus the emissions are known as thermionic emissions. This is what happens inside of working vacuum tubes. The thermionically emitted electrons try to move towards a positively charged electrode (the plate), but my be required to pass by or through other electrodes along the way, depending upon the nature of the tube.