The term surd traces back to al-KhwÄrizmÄ«, who referred to rational and Irrational Numbers as audible and inaudible, respectively. This later led to the Arabic word "أصم‎" (asamm, meaning "deaf" or "dumb") for irrational number being translated into Latin as "surdus" (meaning "deaf" or "mute").
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Only some - a minority - of irrational numbers are called surds. Most of them, such as pi, phi, e are not surds.
Irrational numbers are not rational. They were thought to be "roots that were deaf to reason". The Latin word surdus, (= deaf) led to the term surd.