A stringed musical instrument capable of precise
re-tuning by means of pedals or levers actuating
a mechanism that either increases or decreases
tension on the affected strings.

You're not likely to find the above entry in Websters' dictionary - the omission is present also in music reference books that include classifications of other stringed instruments. It has been stated that the banjo is considered to be the only true American musical instrument - but isn't that a distinction that needs to be revised? Sure, its true that the pedal steel guitar is an adaptation of the Hawaiian steel guitar, but that evolution resulted in a new and distinct instrument with a unique voice of its own.

The modern pedal steel guitar typically consists of two ten string necks mounted on a cabinet that houses the pitch-changing mechanism underneath, which is controlled by foot operated pedals and levers operated by the players' knees when seated at the instrument. Two dominant tunings have emerged since the pedal steels' development in the late 1930's, the E9th and C6th tunings. These are usually on the front and back necks, respectively on a doublenecked guitar, and it should be noted that the universal tuning, which incorporates both the 9th and the 6th tunings on a single neck, is becoming widely used.