In quantum computing, and more specifically in superconducting quantum computing, a transmon is a type of superconducting charge qubit that was designed to have reduced sensitivity to charge noise.
Its name is an abbreviation of the term transmission line shunted plasma oscillation qubit which consists of a Cooper-pair box “where the two superconductors are also capacitatively shunted in order to decrease the sensitivity to charge noise, while maintaining a sufficient anharmonicity for selective qubit control“.
A transmon looks far more like a classic LC circuit. It is often depicted as a Josephson junction in parallel with a very large capacitor and thus it is manipulated using microwave frequencies, not gate voltages.