The term 'TRANSMISSION UNIT' can be broken down into two parts: 'TRANSMISSION' and 'UNIT'. TRANSMISSION is the process of sending and receiving information, such as data or signals, from one place to another. A UNIT is a single thing or person, or a group of things or people that are considered together. Therefore, a TRANSMISSION UNIT is a device or system that is used to send and receive information from one place to another. The crossword solution for this term is 'GEAR', which is a mechanical device that is used to transmit rotational force from one part of a machine to another.