In statistics, a *variable* is something measurable or observable that is expected to either change over time or between individual observations. Examples of variables in statistics include the age of students, their hair colour or a playing field’s length or its shape.

*Numerical variables* are variables whose values are numbers, and for which arithmetic processes such as adding and subtracting, or calculating an average, make sense.

Examples include the number of children in a family or the number of days in a month.

A *discrete numerical variable* is a *numerical variable*, each of whose possible values is separated from the next by a definite ‘gap’. The most common numerical variables have the counting numbers 0,1,2,3,… as possible values. Others are prices, measured in dollars and cents.

In algebra, a *variable* is a symbol, such as *real number*.

A Venn diagram is a graphical representation of the extent to which two or more events, for example A and B, are mutually inclusive (overlap) or mutually exclusive (do not overlap).

A vertex is the point where two line segments or rays meet, join, or intersect.

When two lines intersect, four angles are formed at the point of intersection. In the diagram, the angles marked ∠AOX and ∠BOY are called vertically opposite. Vertically opposite angles are equal.

The *volume* of a *solid* is a measure of the space enclosed by the solid.

For a rectangular prism, *Volume* = *Length* × *Width* × *Height.*