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]