A mouth organ is a generic term for free reed

Free reed aerophone

A free reed aerophone is a musical instrument where sound is produced as air flows past a vibrating reed in a frame. Air pressure is typically generated by breath or with a bellows.- Operation :...

 aerophone

Aerophone

An aerophone is any musical instrument which produces sound primarily by causing a body of air to vibrate, without the use of strings or membranes, and without the vibration of the instrument itself adding considerably to the sound...

 with one or more air chambers fitted with a free reed.

Though it spans many traditions, it is played universally the same way by the musician placing their lips over a chamber or holes in the instrument, and blowing or sucking air to create a sound. Many of the chambers can be played together or each individually.

The mouth organ can be found all around the world and is known by many different names and seen in many different traditions. The most notable variations include the harmonica

Harmonica

The harmonica, also called harp, French harp, blues harp, and mouth organ, is a free reed wind instrument used primarily in blues and American folk music, jazz, country, and rock and roll. It is played by blowing air into it or drawing air out by placing lips over individual holes or multiple holes...

, the pan flute

Pan flute

The pan flute or pan pipe is an ancient musical instrument based on the principle of the closed tube, consisting usually of five or more pipes of gradually increasing length...

, and Asian free reed wind instruments consisting of a number of bamboo pipes of varying lengths fixed into a wind chest; these include the sheng

Sheng (instrument)

The Chinese sheng is a mouth-blown free reed instrument consisting of vertical pipes.Traditionally, the sheng has been used as an accompaniment instrument for solo suona or dizi performances. It is one of the main instruments in kunqu and some other forms of Chinese opera...

, khaen, lusheng

Lusheng

The lusheng is a Chinese musical instrument with multiple bamboo pipes, each fitted with a free reed, which are fitted into a long blowing tube made of hardwood. It most often has five or six pipes of different pitches, and is thus a polyphonic instrument...

, yu

Yu (wind instrument)

The yu was a free reed wind instrument used in ancient China. It was similar to the sheng, with multiple bamboo pipes fixed in a wind chest which may have been made of bamboo, wood, or gourd. Each pipe contained a free reed, which was also made of bamboo. Whereas the sheng was used to provide...

, Shō, and saenghwang

Saenghwang

The saenghwang is a Korean wind instrument. It is a free reed mouth organ derived from the Chinese sheng, though its tuning is different....

. The melodica

Melodica

The melodica, also known as the "blow-organ" or "key-flute", is a free-reed instrument similar to the melodeon and harmonica. It has a musical keyboard on top, and is played by blowing air through a mouthpiece that fits into a hole in the side of the instrument. Pressing a key opens a hole,...

, consisting of a single tube that is essentially blown through a keyboard

Keyboard instrument

A keyboard instrument is a musical instrument which is played using a musical keyboard. The most common of these is the piano. Other widely used keyboard instruments include organs of various types as well as other mechanical, electromechanical and electronic instruments...

, is another variation.