1. A consonant produced without sound from the vocal cords.
2. In mathematics, an irrational root or irrational number, or an expression containing one or the other.
3. 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.
4. Etymology: from Latin surdus, "unheard, silent, dull"; possibly related to susurrus, "a muttering, a whispering".