Why are the quadratic equations of the second degree called quadratic?

Answer It is due to the propensity of scholars of all types to label things of profound importance with Words or modifications of Words of a long dead language. In this case "quadratic" comes from the Latin "quadratus", meaning square.

This is in fact the area of a square of a side "x" is x^2, so every equation having variable with exponent 2 become quadratic equation.