"Good Vibrations" is a song composed by Brian Wilson with words by Mike Love for the American rock band the Beach Boys, of which both were members. Released on October 10, 1966, the single was an immediate critical and commercial hit, topping record charts in several countries including the US and UK. Characterized by its complex soundscapes, episodic structure, and subversions of pop music formula, it was the costliest single ever recorded at the time of its release. "Good Vibrations" later became widely acclaimed as one of the greatest masterpieces of rock music.

Although the song does not technically contain a theremin, "Good Vibrations" is the most frequently cited example of the instrument's use in pop music. Upon release, the single prompted an unexpected revival in theremins and increased the awareness of analog synthesizers. The notion that "Good Vibrations" features a theremin has been erroneously repeated in books, CD liner notes, and quotes from the recording's participants. While having a similar sound, a theremin is an aerial-controlled instrument, unlike the Electro-Theremin.  When the Beach Boys needed to reproduce its sound onstage, Wilson first requested that Tanner play the Electro-Theremin live with the group, but he declined due to commitments. Tanner recalls saying to Wilson, "I've got the wrong sort of hair to be on stage with you fellas", to which Wilson replied: "We'll give you a Prince Valiant wig." The Beach Boys then requested the services of Walter Sear, who asked Bob Moog to design a ribbon controller, since the group was used to playing the fretboards of a guitar. Sears remembers marking fretboard-like lines on the ribbon "so they could play the damn thing." Moog began manufacturing his own models of theremins. He later noted: "The pop record scene cleaned us out of our stock which we expected to last through Christmas."  In Steven M. Martin's 1993 documentary Theremin: An Electronic Odyssey, in which Wilson makes an appearance, it was revealed that the attention being paid to the theremin due to "Good Vibrations" caused Russian authorities to exile its inventor, Leon Theremin.

Using a quote from the above article, answer the following question: what is theremin?
the single prompted an unexpected revival in theremins and increased the awareness of analog synthesizers.