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, §§169, 179, 180.
Surd, n. (Math.) 1. A quantity which can not be expressed by rational
numbers; thus, √2 is a surd.