falsetto

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A voice in a register higher than a person's usual register, resulting from a change in the way the vocal cords are vibrated.
• noun One who sings or talks in this register.

from The Century Dictionary.

• noun The highest or smallest register or quality in both male and female voices: so called because in its untrained state it is more or less unnatural and forced, and because at best it is usually intractable.
• Having the quality and compass of the falsetto.
• Assumed; constrained; unnaturally high-pitched; false.

from the GNU version of the Collaborative International Dictionary of English.

• noun A false or artificial voice; that voice in a man which lies above his natural voice; the male counter tenor or alto voice. See Head voice, under voice.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun countable or uncountable The "false" (singing) voice in any human, usually airy and lacking a purity of vowels; created by utilizing the next highest vocal folds above those used for speech and normal range singing. It is commonly confused with the Head Voice register.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective artificially high; above the normal voice range
• noun a male singing voice with artificially high tones in an upper register