The theremin is…
…is an early electronic musical instrument controlled without physical contact by the thereminist (performer). It is named after the westernized name of its Russian inventor, Léon Theremin, who patented the device in 1928.
The instrument’s controlling section usually consists of two metal antennas which sense the relative position of the thereminist’s hands and control oscillators for frequency with one hand, and amplitude (volume) with the other. The electricsignals from the theremin are amplified and sent to a loudspeaker.
It is essentially played just by moving one’s hands through the air. You might think this is a recent invention what with all our new fangled technology, but it was actually created almost a century ago (1918). Here’s an example of a good theremin player:
And here’s what happens when a cat gets a hold of one: