Examples:

icon  A small picture or diagram, sometimes used to represent quantities.  For example, an icon of a stadium might be used to represent 100,000 people on a pictograph.  Icons are also used to represent functions or objects in computer operating systems and applications.

icosahedron  See regular polyhedron.

image  A figure that is produced by a transformation of another figure called the preimage.

improper fraction  See top-heavy fraction.

inch (in)  In the U.S. customary system, a unit of length equal to 1/12 of a foot.  The inch is defined as 2.54 centimeters.

index of locations  A list of places together with a system for locating them on a map; for example, “Billings D-3,” indicates that Billings will be within a certain rectangle on a map.  The rectangle is under the letter D and to the right of the number 3 on the borders of the map.

indirect measurement  Methods for determining heights, distances, and other quantities that cannot be measured directly.

inequality  A number sentence stating that two quantities are not equal, or might not be equal.  Relation symbols for inequalities include ¹, <, >, <, >.

inscribed polygon  A polygon all of whose vertices are points on a circle or other figure.

integers  The set of integers is {…, -4, -3, -2, -1, 0, 1, 2, 3, 4, …}.  All integers are rational numbers, but not all rational numbers are integers.  All counting numbers are integers, but not all integers are counting numbers.

intersect  To meet (at a point, line, and so on).  Sharing a common point or points.

interval  A set of numbers between two numbers a and b, possibly including a and/or b.

“in the black”  A positive account balance.  You have more money than you owe.

“in the red”  A negative account balance.  You owe more money than you have.

integer  Any whole number or its opposite; for example, -2, 2, 6, -100.

interior  The set of all points in a plane “inside” a closed two-dimensional figure, such as a polygon or circle.  Also, the set of all points in space “inside” a closed three-dimensional figure, such as a polyhedron or sphere.  The interior is usually not considered to be part of the figure.

irrational numbers  Numbers that cannot be written as fractions where both the numerator and denominator are integers and the denominator is not zero.  For example,   and p are irrational numbers.  An irrational number can be represented by a nonterminating, nonrepeating decimal.  For example, the decimal for p, 3.141592653…, continues without a repeating pattern.  The number 1.10100100010000… is irrational; there is a pattern in the decimal, but it does not repeat.

irregular (polygons)  Polygons with sides of different lengths.

isometry transformation  A transformation of an object such as a translation (slide), reflection (flip), or rotation (turn) that preserves distances and orientations between points on the object.

juxtapose  To place side by side in an expression to indicate multiplication.  For example, 5n means 5 * n, and ab means a * b.