[kwo-drat-ik]
1.
2.
Algebra. involving the square and no higher power of the unknown quantity; of the second degree.
noun
3.
a quadratic polynomial or equation.

Origin:
1650–60; quadrate + -ic

Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2013.
Collins
World English Dictionary
 quadratic (kwɒˈdrætɪk) —n 1. Also called: quadratic equation an equation containing one or more terms in which the variable is raised to the power of two, but no terms in which it is raised to a higher power —adj 2. of or relating to the second power

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Etymonline
Word Origin & History

1650s, "square," from quadrate (late 14c.), from L. quadratus "square," pp. of quadrare "to square," related to quattuor "four" (see four). Quadratic equations (1660s) so called because they involve the square of x.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
 quadratic   (kwŏ-drāt'ĭk)  Pronunciation Key  Relating to a mathematical expression containing a term of the second degree, such as x2 + 2. ◇ A quadratic equation is an equation having the general form ax2 + bx + c = 0, where a, b, and c are constants. ◇ The quadratic formula is x = -b ± √(b2 - 4ac)/2a. It is used in algebra to calculate the roots of quadratic equations.
The American Heritage® Science Dictionary