QED

An abbreviation for the Latin "quod erat demonstrandum", meaning "which was to be shown". It is written, or replaced by ∎, at the end of a proof.

Quadrant

One of the four regions of the coordinate plane formed by the x- and y-axes, numbered I to IV counterclockwise starting at the upper right.

Quadrantal angle

An angle in standard position whose terminal side lies on an axis, such as 0°, 90°, 180° or 270°.

Quadratic equation

An equation that can be written as ax² + bx + c = 0, where a is not 0. It can have two, one or no real solutions.

Quadratic formula

A formula that solves any quadratic equation ax² + bx + c = 0: x equals −b plus or minus the square root of b² − 4ac, all divided by 2a.

Quadratic function

A function of the form f(x) = ax² + bx + c with a not 0. Its graph is a parabola.

Quadrilateral

A polygon with four sides, four angles and four vertices.

Quadrillion

One thousand trillion: 1,000,000,000,000,000, or 10¹⁵.

Quadruple

To multiply by four, or to make four times as large.

Qualitative data

Data that describe qualities or categories rather than numbers, such as favorite color or type of pet; also called categorical data.

Quantitative data

Data that are numbers from counting or measuring, such as height, age or test scores.

Quantity

An amount or number of something that can be counted or measured.

Quart

A US customary unit of capacity equal to 2 pints, or ¼ of a gallon, about 0.946 liters.

Quarter

One of four equal parts of a whole, written ¼ or 25%. It also names a 25-cent coin and 15 minutes (a quarter of an hour).

Quarterly

Happening four times a year, once every three months.

Quartic polynomial

A polynomial of degree 4, such as x⁴ − 3x² + 2.

Quartiles

The three values that divide ordered data into four equal parts: the first quartile (Q1), the median (Q2) and the third quartile (Q3).

Quintic

A polynomial of degree 5, where the degree is the highest exponent, such as x⁵ + 4x³ − 6.

Quintuple

To multiply by five, or to make five times as large.

Quotient

The result of dividing one number by another.

Quotient rule

For exponents, dividing powers with the same base subtracts the exponents: xᵃ ÷ xᵇ = xᵃ⁻ᵇ. In calculus, a rule for the derivative of one function divided by another.