surd

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

• noun Mathematics An irrational number, such as √2.
• noun Linguistics A voiceless sound in speech.
• adjective Voiceless, as a sound.

from The Century Dictionary.

• To render dim or soft; mute.
• Containing or involving a surd : thus [1 + ½] ½ is a surd expression but not a surd, since 1 + ½ is not a rational expression.
• noun In mathematics: An indicated root whose value is irrational, but whose radicand is rational, as ½. A surd is quadratic, cubic, of order n, according as its exponent is ½, ⅓, 1/n;.
• Not having the sense of hearing; deaf.
• That cannot be discriminated by the ear (?).
• In mathematics, not capable of being expressed in rational numbers: as, a surd expression, quantity, or number. See II., 1.
• In phonetics, uttered with breath and not with voice; devoid of vocality; not sonant: toneless: specifically applied to the breathed or non-vocal consonants of the alphabet. See II., 2.
• Meaningless; senseless.
• noun In mathematics, a quantity not expressible as the ratio of two whole numbers, as √ 2, or the ratio of the circumference of a circle to the diameter.
• noun In phonetics, a consonantal sound uttered with breath and not with voice; a non-sonant consonant; a non-vocal alphabetic utterance, as p, f, s, t, k, as opposed to b, v, z, d, g, which are sonants or vocals.

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

• noun (Math.) A quantity which can not be expressed by rational numbers.
• noun (Phon.) A surd element of speech. See Surd, a., 4.
• adjective obsolete Net having the sense of hearing; deaf.
• adjective obsolete Unheard.
• adjective (Math.) Involving surds; not capable of being expressed in rational numbers; radical; irrational.
• adjective (Phonetics) Uttered, as an element of speech, without tone, or proper vocal sound; voiceless; unintonated; nonvocal; atonic; whispered; aspirated; sharp; hard, as f, p, s, etc.; -- opposed to sonant. See Guide to Pronunciation, §§169, 179, 180.

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

• noun arithmetic An irrational number, especially one expressed using the √ symbol.
• noun linguistics A voiceless consonant.

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

• noun a consonant produced without sound from the vocal cords
• adjective produced without vibration of the vocal cords