# Dictionary Definition

quadric n : a curve or surface whose equation (in Cartesian coordinates) is of the second degree [syn: quadric surface]

# Extensive Definition

In mathematics, a quadric, or quadric surface, is any D-dimensional hypersurface defined as the locus of zeros of a quadratic polynomial. In coordinates \, the general quadric is defined by the algebraic equation
\sum_^D Q_ x_i x_j + \sum_^D P_i x_i + R = 0
where Q is a (D + 1)×(D + 1) matrix and P is a (D + 1)-dimensional vector and R a constant. The values Q, P and R are often taken to be real numbers or complex numbers, but in fact, a quadric may be defined over any ring. In general, the locus of zeros of a set of polynomials is known as an algebraic variety, and is studied in the branch of algebraic geometry.
A quadric is thus an example of an algebraic variety. For the projective theory see quadric (projective geometry).
The normalized equation for a three-dimensional (D=3) quadric centred at the origin (0,0,0) is:
\pm \pm \pm =1.
Via translations and rotations every quadric can be transformed to one of several "normalized" forms. In three-dimensional Euclidean space there are 16 such normalized forms, and the most interesting, the nondegenerate forms are given below. The remaining forms are called degenerate forms and include planes, lines, points or even no points at all.
In real projective space, the ellipsoid, the elliptic paraboloid and the hyperboloid of two sheets are equivalent to each other up to a projective transformation; the hyperbolic paraboloid and the hyperboloid of one sheet are not different from each other (these are ruled surfaces); the cone and the cylinder are not different from each other (these are "degenerate" quadrics, since their Gaussian curvature is zero).
In complex projective space all of the nondegenerate quadrics become indistinguishable from each other.