A brass instrument is a musical instrument whose tone is produced by vibration of the lips as the player blows into a tubular resonator. They are also called labrosones, literally meaning "lip-vibrated instruments" (Baines, 1993).
To effectively change pitch on a valved brass instrument, two things come into play: the pressing of valves to effectively change the length of the tubing, and the player's lip aperture or "embouchure" setting, which determines the frequency of the pulsed air or vibrations into the instrument.
The view of most scholars (see organology) 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 many of woodwind instruments are made of brass, like the saxophone.