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### A Glossary of Mathematical Terms - O

#### Obtuse

See Angle or Triangle.

#### Octagon

An eight-sided polygon.

#### Odd number

An integer number that divided by 2 gives a remainder of 1.

#### One-to-one correspondence

A map where to not only to every element in the first set there corresponds one and only one element in the second set, but also vice-versa.

#### Operation

A specific process for combining quantities.

Two numbers whose sum is 0.

#### Order of operations

The convention that detremines in which oder an expression is to be evaluated. A common mnemonic to help folks to remember the correct order for mathematical expressions is: Please excuse my dear aunt Sally. Parentheses, exponents, multiplication, division , addition, subtraction.

#### Ordered pair

A pair of objects in which the order is important.

#### Ordinal number

A number that describes order or position.

#### Ordinate

The second element in a coordinate pair.

first published on september 01 2003 - last updated on september 01 2003