decagon: a polygon with ten sides.
decimal numbers: the numbers in the base 10 number system.
decreasing function: a function is considered to be decreasing if f(b) < f(a) when b > a.
deduction: a conclusion arrived at by reasoning.
definite integral: the definite integral of f(x) between a and b represents the area under
the
curve y = f(x) , above the x  axis, to the right
of the line x = a, and to
the
left of the line x = b. The definite integral of f(x)
= F(b)  F(a)
where F is an antiderivative function for f(x).
degree: 1/360 of a full rotation. There are
360 degrees in a circle. Unit of measure of an
angle.
denominator: the bottom part of a fraction.
dependent variable: the output of a function.
derivative: the rate of change
of a function. The derivative at x of f(x) is the slope of the
tangent
line at (x, f(x)). y' = f ' (x) = [f(x + delta x)  f(x)]
/ delta x.
determinant: the determinant  a b  = ad  bc.

c d 
diagonal: the line segment connecting two nonadjacent vertices in a polygon.
diameter: the line segment joining two points on a circle and passing through the center.
difference: the result of subtracting two numbers.
differentiable: a function is differentiable over an interval
if it is continuous over the
interval
and if the derivative exists everywhere on the interval.
differential: an infinitesimally small change in a variable, represented by d, as in dx, or dy.
differentiation: the process of finding a derivative.
digit: the ten symbols, 0, 1, 2, 3, 4, 5, 6, 7, 8, and
9 are digits. Example: the number 365
has
three digits: 3, 6, and 5.
dimension: The dimension of a space is the number of coordinates needed to identify a
location in that space.
directly proportional: y is directly proportional to x if y = kx.
discriminant: the discriminant of a quadratic equation, ax^{2} + bx + c = 0 is b^{2}  4ac.
The discriminant tells how many roots there are for the equation and the
nature of the roots.
disjoint: having no elements in common.
disjunction: an OR statement.
distributive property: a(b + c) = ab + ac
divergent series: a series whose sum is infinite.
dividend: In a / b = c, a is the dividend.
division: the opposite operation of multiplication.
divisor: In a / b = c, b is the divisor.
dodecahedron: a polyhedron with twelve faces.
domain: the set of all possible values of the argument of a function.