5 is a surd.
A surd expression or quantity; a surd number.
Origin of surd
- Medieval Latin surdus speechless, surd (translation of Arabic (jaḏr) ’aṭamm deaf (root), surd) (translation of Greek alogos speechless, surd) from Latin
From American Heritage Dictionary of the English Language, 5th Edition
- Latin surdus (“deaf"); in mathematical sense, "deaf to reason", i.e. irrational.