Data
Facts or information collected, such as counts or measurements, often organized in tables or graphs.
Decade
A period of 10 years.
Decagon
A polygon with 10 sides and 10 angles.
Decay factor
In exponential decay, the number between 0 and 1 that a quantity is multiplied by in each time period, such as 0.5 when an amount halves.
Decimal number
A number written in the base-ten system with a decimal point, such as 3.75, which has a whole-number part and a part less than one.
Decimal places
The positions to the right of the decimal point: tenths, hundredths, thousandths and so on.
Decimal point
The dot in a decimal number that separates the whole-number part from the part less than one.
Decimeter
A metric unit of length equal to one tenth of a meter, or 10 centimeters.
Decompose
To break a number or shape into smaller parts, such as writing 47 as 40 + 7.
Decreasing function
A function whose output values go down as the input values go up.
Deductive reasoning
Reaching a conclusion that must be true by applying general rules or facts to a specific case.
Definite integral
An integral with limits a and b. It gives the net area between a curve and the x-axis from x = a to x = b.
Degree (angle)
A unit for measuring angles, equal to 1/360 of a full turn.
Degree of a polynomial
The highest exponent of the variable in a polynomial. The degree of 4x³ + 2x² − 7 is 3.
Denominator
The number below the fraction bar. It tells how many equal parts make a whole.
Density
The mass of a substance per unit of volume, D = m / V. It can also mean the number of people or things per unit of area.
Dependent events
Events where the outcome of one changes the probability of the other, such as drawing two marbles without putting the first one back.
Dependent variable
The variable whose value depends on another variable. It is usually y, shown on the vertical axis.
Derivative
The instantaneous rate of change of a function, which equals the slope of the tangent line at a point. It is written f′(x).
Descending order
An arrangement of numbers or items from largest to smallest.
Determinant
A number calculated from a square matrix. For a 2×2 matrix with rows a b and c d, it is ad − bc.
Deviation
How far a data value is from the mean. Values above the mean have positive deviations and values below have negative ones.
Diagonal
A line segment joining two vertices of a polygon that are not next to each other.
Diameter
A chord that passes through the center of a circle; its length is twice the radius.
Difference
The result of subtracting one number from another.
Difference of squares
An expression of the form a² − b², which factors as (a + b)(a − b).
Digit
Any of the ten symbols 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9 used to write numbers.
Dilation
A transformation that enlarges or shrinks a figure by a scale factor from a fixed center point.
Dimensional analysis
Converting units by multiplying by conversion factors so that unwanted units cancel.
Dimensions
The measurements of a figure, such as its length, width and height.
Direct variation
A relationship in which one quantity is a constant multiple of another, y = kx. Its graph is a straight line through the origin.
Directrix
A fixed line used with a focus to define a parabola. Every point on the parabola is the same distance from the focus and the directrix.
Discount
The amount taken off the regular price of an item.
Discount rate
The percent by which a price is reduced.
Discrete data
Data that can only take separate, countable values, such as the number of siblings or goals scored.
Discriminant
The expression b² − 4ac from the quadratic formula. It tells whether a quadratic equation has two, one or no real solutions.
Disjoint sets
Sets that have no elements in common. Their intersection is the empty set.
Distance formula
A formula for the distance between two points: d = √((x₂ − x₁)² + (y₂ − y₁)²).
Distributive property
The rule that multiplying a sum by a number gives the same result as multiplying each addend and then adding: a(b + c) = ab + ac.
Dividend
The number being divided in a division problem.
Divisibility
Whether one whole number can be divided by another with no remainder. Divisibility rules let you check quickly.
Divisibility rules
Shortcuts for telling whether a number divides evenly by another without doing the division.
Divisible
Able to be divided by another number with no remainder. 15 is divisible by 5.
Division
Splitting a quantity into equal groups, or finding how many times one number is contained in another.
Divisor
The number you divide by in a division problem.
Dodecagon
A polygon with 12 sides and 12 angles.
Dodecahedron
A polyhedron with 12 faces. In a regular dodecahedron, every face is a regular pentagon.
Domain
The set of all input values (x-values) for which a function is defined.
Dot plot
A graph that shows each data value as a dot above a number line, stacking dots for repeated values.
Double number line
Two parallel number lines with aligned marks, used to show equivalent ratios.
Dozen
A group of 12.