What does "surd" mean?

Additional senses

1. 2.(Math.) Involving surds; not capable of being expressed in rational numbers; radical; irrational; as, a surd expression or quantity; a surd number.
2. 3.(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.
3. 4.A quantity which can not be expressed by rational numbers; thus, *2 is a surd.
4. 5.(Phon.) A surd element of speech. See Surd, a., 4.