A brass instrument is a musical instrument whose sound is produced by sympathetic vibration of air in a tubular resonator in sympathy with the vibration of the player's lips. Brass instruments are also called labrosones, literally meaning "lip-vibrated instruments".
There are two factors in producing different pitches on a brass instrument: One is alteration of the player's lip tension (or "embouchure"), and air flow. The second is the use of slides or valves to change the length of the tubing, thus changing the harmonic series presented by the instrument to the player.
The view of most scholars is that the term "brass instrument" should be defined by the way the sound is made, as above, and not by whether the instrument is actually made of brass. Thus, as exceptional cases one finds brass instruments made of wood like the alphorn, the cornett, and the serpent, while some woodwind instruments are made of brass, like the saxophone.