An aerophone is a musical instrument that produces sound primarily by causing a body of air to vibrate, without the use of strings or membranes (which are respectively chordophones and membranophones), and without the vibration of the instrument itself adding considerably to the sound (or idiophones).

According to Curt Sachs:

Aerophones or 'air instruments' include what are usually called 'wind instruments,' with the addition of a few instruments with a different acoustical principle called 'free aerophones.'
A wind instrument has two essential factors: a tube enclosing a column of air, and a device for setting that air into vibration by interrupting into pulsations the steady breath of the player (or the wind of a bellows).

These may be lips, a mechanical reed, or a sharp edge. Also, an aerophone may be excited by percussive acts, such as the slapping of the keys of a flute or of any other woodwind. A free aerophone lacks the enclosed column of air yet, "cause a series of condensations and rarefications by various means."

Aerophones are one of the four main classes of instruments in the original Hornbostel–Sachs system of musical instrument classification, which further classifies aerophones by whether or not the vibrating air is contained within the instrument. The first class (41) includes instruments which, when played, do not contain the vibrating air. The bullroarer is one example. These are called free aerophones. This class includes (412.13) free reed instruments, such as the harmonica, but also many instruments unlikely to be called wind instruments at all by most people, such as sirens and whips. The second class (42) includes instruments that contain the vibrating air when being played. This class includes almost all instruments generally called wind instruments — including the didgeridoo, (423) brass instruments (e.g., trumpet, french horn, baritone horn, tuba, trombone), and (421 & 422) woodwind instruments (e.g., oboe, flute, saxophone, clarinet). The wind factor is not only provided by the players' lungs function. The organ and the mouth harmonica are also aerophones, both supplied with free reeds, which are blown by a mechanical system.

Additionally, very loud and impulsive sounds can be made by explosions directed into, or being detonated inside of resonant cavities. Detonations inside the calliope (and steam whistle), as well as the pyrophone, might thus be considered as class 42 instruments, despite the fact that the "wind" or "air" may be steam or an air-fuel mixture.

Other cases of aerophones with impulsive sounds are the boomwhackers and the so-called thongophones, made up of cylindrical pipes that are struck on the sides (boomwhackers) or extremes (thongophones), thus generating percussive aerophonic tones.^{[citation needed]}

According to Ardal Powell, the flute is a simple instrument found in numerous ancient cultures. There are three legendary and archeologically verifiable birthplace sites of flutes: Egypt, Greece and India. Of these, the transverse flute (side-blown) appeared only in ancient India, while the fipple flutes are found in all three. It is likely, states Powell, that the modern Indian bansuri has not changed much since the early medieval era.