Undecagon
A polygon with 11 sides and 11 angles; also called a hendecagon.
Undefined
Having no meaning or value. Dividing by zero, as in 5 ÷ 0, is undefined because no number times 0 gives 5.
Undefined slope
The slope of a vertical line. The run is 0, so rise ÷ run would mean dividing by zero.
Unequal
Not having the same value. The symbol ≠ means "is not equal to," as in 3 + 4 ≠ 8.
Unequal parts
Pieces of a whole that are not the same size. A whole must be cut into equal parts to show fractions like halves or fourths.
Uniform probability
A probability model in which every outcome is equally likely, such as rolling a fair number cube, where each number has a probability of 1/6.
Unimodal
Describes a data set or graph that has exactly one mode, or one peak.
Union
The set of all elements that belong to either of two sets or to both, written A ∪ B.
Unique solution
Exactly one answer. A system of two linear equations has a unique solution when its lines cross at a single point.
Unit
A fixed amount used as a standard for measuring, such as an inch, a gram, a liter, or a second.
Unit circle
A circle with a radius of 1 centered at the origin. The point where an angle θ meets it has coordinates (cos θ, sin θ).
Unit conversion
Changing a measurement from one unit to another without changing the amount, for example 2 ft = 24 in.
Unit cube
A cube whose edges are each 1 unit long. Its volume is 1 cubic unit, and it is used to measure volume.
Unit fraction
A fraction with 1 as its numerator, such as 1/2, 1/3, or 1/8.
Unit price
The price of one unit of an item, found by dividing the total price by the number of units. It helps you compare the cost of different package sizes.
Unit rate
A rate in which the second quantity is 1 unit, such as 60 miles per 1 hour.
Unit square
A square with sides 1 unit long. Its area is 1 square unit, and it is used to measure area.
Unit vector
A vector with a length (magnitude) of 1. It shows a direction only.
Univariate data
Data that describes a single variable, such as the heights of the students in a class.
Universal set
The set of all elements being considered in a problem. It is drawn as the rectangle around a Venn diagram and labeled U.
Unknown
A value that is not known yet, usually shown by a letter such as x, that you solve for.
Unknown term
The missing number in a proportion, often written as a letter. In 3/4 = x/12, the unknown term is x, and x = 9.
Unlike denominators
Denominators that are different, as in 1/2 and 1/3. You need a common denominator before you can add or subtract such fractions.
Unlike fractions
Fractions with different denominators, such as 1/2 and 1/3, so their pieces are different sizes.
Unlike terms
Terms with different variables, or the same variables raised to different powers, such as 3x and 4x². Unlike terms cannot be combined.
Unlikely event
An event with a low chance of happening; its probability is closer to 0 than to 1.
Upper bound
A number that is greater than or equal to every number in a set. In measurement, it is the largest value that would round to the measured amount.
Upper extreme
The greatest value in a data set. On a box-and-whisker plot it is the end of the right whisker.
Upper quartile
The median of the upper half of a data set, also called the third quartile (Q3). About 75% of the data are at or below it.
US customary units
The system of measurement used in the United States, with units such as inches, feet, miles, ounces, pounds, cups, and gallons.