State or produce the name of a displayed numeral.

Plural of index. When the product of *a* × *a* × *a* is written as *a*^{3} the number 3 is called the index, often also referred to as the ‘power’ or the ‘exponent’.

Indirect comparison is the process of comparing two objects that cannot be directly aligned, such as the length of two fixed benches.

Two quantities are in indirect proportion (sometimes called inverse proportion) if as one increases, the other decreases by the same percentage. An example of an indirect proportion is the lengths and widths of rectangles with the same area. As the length doubles the width must halve to maintain the same area.

A zero digit in a numeral in a position other than the initial or final position, usually where the following digits are not all zero. For example, 1010 has one internal zero in the hundreds position but 1000 has none.

Inverse operations in arithmetic undo the effect of an operation. For example, multiplication and division are inverse operations as are addition and subtraction.

Repeating a process. Iterating a unit of length involves repeatedly using one unit, noting where it ends, then moving it to align the end point with the beginning of the next iteration of the unit.