A musical instrument that contains some type of resonator (usually a tube), in which a column of air is set into vibration by the player blowing into (or over) a mouthpiece set at the end of the resonator. The pitch of the vibration is determined by the length of the tube and by manual modifications of the effective length of the vibrating column of air.

Wind instruments fall into one of 2 categories: brass and woodwind. Although brass instruments were originally made of brass and woodwind instruments have traditionally been made of wood, the material used to make the body of the instrument is not always a reliable guide to its family type.

A more accurate way to determine whether an instrument is brass or woodwind is to examine how the player produces sound. In brass instruments, the player's lips vibrate, causing the air within the instrument to vibrate. In woodwind instruments the player either:

1. causes a reed to vibrate, which agitates the column of air (as in a clarinet or oboe)
2. blows against an edge or fipple (as in a recorder) or
3. blows across the edge of an open hole (as in a flute).

For example, the saxophone, flute, and metal clarinet are all typically made of brass, but are classified as woodwind instruments due to the method of vibrating the air column (except for the flute these all use a reed). On the other hand, the wooden cornett (not to be confused with the cornet, which is made of brass) and the serpent are both made of wood, but belong to the family of brass instruments because the vibrating is done by the player's lips. The harmonica is also considered a woodwind instrument for this purpose.