The Collaborative International Dictionary of English v.0.48:

Surd \Surd\, n. (Math.)
   1. A quantity which can not be expressed by rational numbers;
      thus, [root]2 is a surd.
      [1913 Webster]

   2. (Phon.) A surd element of speech. See Surd, a., 4.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Surd \Surd\, a. [L. surdus deaf (whence the meaning, deaf to
   reason, irrational), perhaps akin to E. swart. Cf.
   Sordine.]
   1. Net having the sense of hearing; deaf. [Obs.] "A surd . .
      . generation." --Sir T. Browne.
      [1913 Webster]

   2. Unheard. [Obs.] --Kenrick.
      [1913 Webster]

   3. (Math.) Involving surds; not capable of being expressed in
      rational numbers; radical; irrational; as, a surd
      expression or quantity; a surd number.
      [1913 Webster]

   4. (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, [sect][sect]169, 179, 180.
      [1913 Webster]

WordNet (r) 3.0 (2006):

surd
    adj 1: produced without vibration of the vocal cords; "unvoiced
           consonants such as `p' and `k' and `s'" [syn: unvoiced,
           voiceless, surd, hard] [ant: soft, sonant,
           voiced]
    n 1: a consonant produced without sound from the vocal cords
         [syn: surd, voiceless consonant]

Moby Thesaurus II by Grady Ward, 1.0:

202 Moby Thesaurus words for "surd":
   Gaussian integer, accented, algebraic number, algorismic,
   algorithmic, aliquot, allophone, alveolar, apical, apico-alveolar,
   apico-dental, articulated, articulation, aspiration, assimilated,
   assimilation, back, barytone, bilabial, broad, cacuminal, cardinal,
   cardinal number, central, cerebral, check, checked, close,
   complex number, consonant, consonantal, continuant, deaf,
   deaf and dumb, deaf-eared, deaf-mute, deafened, decimal,
   defective number, dental, differential, digital, diphthong,
   dissimilated, dissimilation, dorsal, dull-eared, earless,
   epenthetic vowel, even, even number, explosive, exponential,
   figural, figurate, figurative, finite, finite number, flat,
   fraction, fractional, front, glide, glossal, glottal,
   glottalization, guttural, hard, hard of hearing, heavy, high,
   imaginary, imaginary number, impair, impossible, infinite,
   infinity, integer, integral, intonated, irrational,
   irrational number, labial, labialization, labiodental, labiovelar,
   laryngeal, lateral, lax, light, lingual, liquid, logarithmic,
   logometric, low, manner of articulation, mid, mixed number,
   modification, monophthong, monophthongal, morphophoneme, mute,
   muted, narrow, nasal, nasalized, negative, numeral, numerary,
   numerative, numeric, occlusive, odd, open, ordinal, oxytone, pair,
   palatal, palatalized, parasitic vowel, peak, pharyngeal,
   pharyngealization, pharyngealized, phone, phoneme, phonemic,
   phonetic, phonic, pitch, pitched, plosive, polygonal number,
   positive, possible, posttonic, prime, prime number,
   prothetic vowel, pure imaginary, radical, rational,
   rational number, real, real number, reciprocal, rectangular number,
   retroflex, round number, rounded, segmental phoneme, semivowel,
   serial number, soft, sonant, sonority, speech sound, stone-deaf,
   stop, stopped, stressed, strong, stunned, submultiple, syllabic,
   syllabic nucleus, syllabic peak, syllable, tense, thick,
   thick of hearing, throaty, tonal, tone-deaf, tonic, transcendental,
   transcendental number, transfinite number, transition sound,
   triphthong, twangy, unaccented, unhearing, unrounded, unstressed,
   velar, vocable, vocalic, vocoid, voice, voiced, voiced sound,
   voiceless, voiceless sound, voicing, vowel, vowellike, weak,
   whole number, wide, word-deaf