The term 'Toll Units for Semis' refers to the number of AXLES on a semi-truck. The number of AXLES determines the amount of tolls that must be paid when crossing certain bridges or highways. The crossword solution for this term is 'AXLES, ' which is a five-letter word. AXLES are the components of a vehicle that connect the wheels to the frame and allow the vehicle to move. They are essential for semi-trucks, as they provide the necessary support for the heavy loads they carry.