\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\, 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.