The Latin prefix quadri- is used to indicate the number 4, for example, quadrilateral, quadrant,
etc. However, it also very commonly used to denote objects involving the number 2. This is the case because quadratum is the Latin word
for square, and since the area of a square of side length
is given by 
, a polynomial equation having exponent two is known as a
quadratic ("square-like") equation.
By extension, a quadratic surface is a second-order
algebraic surface.
By analogy, since the volume of a cube of side length 
is 
,
a polynomial equation having exponent three is called a cubic
equation. An equation of degree four is then unimaginatively called a quartic
equation, or sometimes (more commonly in older sources) a biquadratic equation.
