The term 'thermionic tube part' refers to the ANODE, which is a five-letter word. An ANODE is an electrode in a thermionic tube that is positively charged and attracts electrons. It is typically made of metal or another conductive material. The ANODE is one of the two main components of a thermionic tube, the other being the cathode. The ANODE is responsible for collecting the electrons emitted by the cathode, allowing them to flow through the tube and create a current.