The Infinite Guitar is a modified electric guitar created by Michael Brook, as a way of allowing a note to be held with infinite sustain (hence the name). It consists of an electronic circuit that takes the signal from a standard guitar pickup, amplifies it, and feeds it back into a separate pickup coil. When set up and used correctly, the result is a continuous sustained note that can be used as is, or treated to create new sounds or emulate traditional instruments.
In addition to his own instrument, based on a Tokai copy of a Stratocaster, Brook produced two other Infinite Guitars. One belongs to Daniel Lanois, and the other belongs to The Edge of U2, who famously used it to record "With or Without You" from 1987's The Joshua Tree.