Lev Sergeyevich Termen, better known as Leon Theremin, was a Russian inventor, most famous for his invention of the theremin, one of the first electronic musical instruments and the first to be mass-produced. He also worked on early television research. His secret listening device, "The Thing", hung for seven years in plain view in the United States ambassador's Moscow office and enabled Soviet agents to eavesdrop on secret conversations.

The theremin is an electronic musical instrument controlled without physical contact by the performer. It is named after its inventor, Leon Theremin, who patented the device in 1928.