Title 
Description 

Decagon 
A 10sided polygon is called a decagon. 

Decreasing 
Decreasing means to lessen in extent or scope, to be reduced. A function is considered to be Decreasing if the values in the range decrease as the values from the domain increase. 

Deductive Logic 
Deductive Logic is employed before events have transpired, before the fact. 

Definite Integral 
An integral evaluated between limits of integration is termed a Definite Integral. 

Degree, Angle 
One 360th of a full rotation is an angle of one degree. 

Degree, Polynomial 
The Degree of a polynomial is the order, or highest power (term) of the polynomial. 

Delta 
Delta is the fourth letter of the Greek alphabet. Uppercase Delta looks like a triangle and is used to mean "the change in..." 

DeMoivre's Theorem 
This theorem allows quick calculations of powers and roots of complex numbers expressed in trigonometric form. 

Denominator 
The Denominator of a fraction is the number on the bottom; it is the divisor of the numerator. 

Dependent Variable 
If y = f(x), then y is a function of x and y is the Dependent Variable. Think of it this way: whatever we get for output "y" depends on the input "x" we grab from the domain of the function. 

Derivative 
A first Derivative is the slope of the line tangent to a function. A Derivative provides an instantaneous rate of change between variables. 

Determinant 
A Determinant is a number associated with a square matrix. It may also be a cofactor, a number associated with a square array from a larger matrix. 

Diagonal 
Convex polygons have Diagonals from each vertex to each nonadjoining vertex. 

Diagonal Matrix 
A square matrix with zero values everywhere except on the main diagonal (upper left to lower right) is termed a Diagonal Matrix. 

Difference 
The result of subtraction is often considered a Difference. 

Differentiable 
If a function is smooth and continuous it is differentiable. 

Differential Equation 
A Differential Equation employs derivatives and algebra to solve for variables that represent functions. 

Digit 
Each of the numerals 0 through 9 is a Digit. The term also refers to place value, as the "tens digit" or the "hundredths digit." 

Dilation 
To grow in size is to dilate, or to undergo Dilation. Most often it means to increase proportionally in all dimensions, but not strictly. Sometimes Dilation is expansion in one dimension only. 

Dimension 
A line has one Dimension. A plane has two Dimensions. A threedimensional object occupies space. 

Dimension, Matrix 
The Dimension of a matrix is its order, or size. We label the order of a matrix by its number of rows then its number of columns. A 4x3 matrix is read as "a four by three matrix" and has four rows and three columns. 

Direct Proportion 
When variables are in Direct Proportion to one another they have the relation that as one variable grows the other either increases or decreases by a constant multiplication factor. When y = kx, we say the variables are in Direct Proportion. 

Direct Variation 
Also direct proportion, Direct Variation describes the relation y = kx. 

Directrix 
A line specific to conic sections hyperbolas, parabolas, and ellipses, known as a Directrix, serves to describe along with the location of the focus (or foci) the loci (points) on the graph of the function. 

Discontinuity 
When a function is literally not continuous because of a gap, a step, a hole, or any kind of "break" it is considered discontinuous. 

Discrete Function 
When the inputs from the domain of the function are not smooth and continuous but rather incremental, the function is considered to be a Discrete Function. 

Discriminant 
In the Quadratic Formula, the radicand (the business inside the squareroot sign) is the Discriminant. In general, a Discriminant provides algebraic information about the roots of polynomials. 

Disjoint 
Disjoint sets have no common elements. 

Disk 
A Disk is most often a circular object with a relatively thin measure in the direction orthogonal to the plane of the circular bases. 

Distance 
A length from one point to another is considered a Distance. Any measurement in one dimension confers a length, which is Distance. 

Distance Formula 
The familiar Distance Formula in Cartesian (rectangular) coordinates is a version of the Pythagorean Theorem, where the sum of the squares of the legs of a right triangle is equal to the square of the hypotenuse. 

Distributive Property 
The familiar Distance Formula in Cartesian (rectangular) coordinates is a version of the Pythagorean Theorem, where the sum of the squares of the legs of a right triangle is equal to the square of the hypotenuse. 

Divergent Geometric Progression 
An infinite geometric progression (or a significant portion of one) is termed Divergent when its common ratio has an absolute value less than or equal to 1, or greater than or equal to 1. 

Dividend 
When we divide, we typically "begin" with a dividend. We divide the dividend by the divisor and we get the resulting quotient. In a fraction, which is always topdividedbybottom (numerator divided by denominator), the top of the fraction is the dividend, the bottom is the divisor, and the value of the resulting fraction is the quotient. 

Divisor 
The number we "take into" the dividend when we divide is termed the Divisor. In fractions, which are always topdividedbybottom (numerator divided by denominator) we divide the top (the dividend) by the bottom (the divisor) and the value of the resulting fraction is the quotient. 

Dodecagon 
A 12sided polygon is a Decagon. 

Dodecahedron 
A 12faced polyhedron is called a Dodecahedron. 

Domain 
The values that are "legal" and "legitimate" to put into a function are the elements of the Domain of that function. When y = f(x), the legitimate values of x are the Domain of the function. 

Dot Product 
A product of vector multiplication, the Dot Product is a scalar, which means it has magnitude only and not an associated direction. The Dot Product does not result in another vector. 

Double 
Twice the value of a real number is Double the value. To Double is to multiply by two, so to Double a half results in a whole. 

Doubling Time 
The time it takes an exponential or geometric growth to double in size (grow by 100 percent of the original value) is its doubling time. 
