Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

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

   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.

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

   2. (Phon.) A surd element of speech. See {Surd}, a., 4.

Source : WordNet®

surd
     adj : of speech sounds [syn: {voiceless}, {unvoiced}] [ant: {sonant}]
     n : a consonant produced without sound from the vocal cords
         [syn: {voiceless consonant}]