The Latin meaning of the word "Surd" is deaf or mute. In earlier days, Arabian mathematicians called rational numbers and irrational numbers as audible and inaudible. Since surds form are made of irrational numbers, they were referred to as asamm (deaf, dumb) in Arabic language, and were later translated in Latin as surds.
Surd is simply used to refer to a number that does not have a root.
√4, 3√8, √25 have roots as answers.
But √6, 3√2, √20 do not have proper roots.
These number forms are termed as surds.