Altissimo is a musical term referring to range and frequency of pitch, specifically, very high notes. Some definitions are very specific about which notes are altissimo and which instruments play them, however. When not referring to any specific instrument, altissimo notes are those notes starting on the G in the octave above the treble clef, or any note G6 or higher.
Some instruments, such as a piccolo, produce these pitches fairly easily. They generally are beyond the range of all but the soprano or sopranino members of instrument families. These notes often are referred to as the "whistle register" for vocalists, although "whistle" tone production involves distinct physiology that is required at different pitch points for different individuals and therefore may happen before G6. Although all voice types have an upper whistle register they may be able to reach with proper training, only sopranos, or women with the highest voice class, are capable of producing true altissimo of G6 and above.
A slightly looser definition considers altissimo notes to be the highest notes of which any instrument is capable of producing, with some people limiting altissimo only to woodwind instruments. This definition refers to pitches above the generally accepted range of the instrument. For example, on the oboe, G5 is considered the top pitch, but oboists are exploring new fingerings and techniques to achieve pitches higher than this. Playing in this range thus is somewhat of an extended or advanced technique.
Altissimo performance creates distinct problems for a player or singer. First, for instrumentalists, the pitches are very hard to keep in tune, and although fingerings usually are based on the harmonic system of the instrument, fingerings can be somewhat awkward and tough to produce with facility. Secondly, instrumentalists also have to establish muscle memory of the exact pressure and embouchure shape necessary to get the instrument to speak, which takes time and extensive practice.
For instrumentalists, it is not solely the performer that determines whether altissimo is possible. The instrument itself matters. Performers have to find the right mouthpiece and reed combinations that suit their mouths. Additionally, they must keep their instrument in excellent condition, as any leaks of air can prevent the proper fingering from working the way it should. Some players find they can reach extreme upper pitches much more easily simply by adjusting these factors.
Unlike instrumentalists who can experiment somewhat with fingerings to produce different pitches, vocalists naturally are somewhat limited by the shape and length of their vocal cords and supporting tissues. For many vocalists, the vocal mechanisms simply are not constructed in a way that allows reaching the altissimo notes well. The amount of air pressure required to get the vocal cords to vibrate is tiring, so extended pitches may only appear once or twice in an entire work.
Vocalists who can produce a clear whistle pitch are considered a rarity in the musical world and may become known for their ability to reach the extreme upper part of their range. Probably one of the most famous examples of a singer for which this was true was the soprano Lucrezia Aguiari, commonly known as La Bastardella, who sang for Wolfgang Amadeus Mozart and who was said to be capable of reaching a C7. More modern vocalists who are known for their whistle notes include Mariah Carey and Georgia Brown; Brown claimed to have reached a G10, but no recording of proof exists. The ability to produce pitches around the true altissimo starting point, G6, is much more common — the famous aria "Der Hölle Rache" from Mozart's "Die Zauberflöte," for instance, requires the Queen of the Night's character to have a good F6.