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O

Oblong

Sometimes used to describe a non-square rectangle.

Obtuse Angle

An angle greater than 90 degrees but less than 180 degrees.

Octagon

A polygon with eight sides. Adjective: octagonal, having the form of an octagon.

Octahedron

A polyhedron (3D polygon) with eight faces. A regular octahedron has faces that are equilateral triangles.

Odd Number

An integer that cannot be divided neatly into two integers. Examples: 1, 3, 5, 7, etc.

Origin

A fixed point from which measurements are taken. See also Cartesian co-ordinate system. On a graph the origin is normally given by the point at which the x axis meets the y axis, at the co-ordinate (0,0).

Ounce

Symbol: oz. An imperial unit of mass. In the imperial system, 16 ounces = 1 pound. 1 ounce is just over 28 grams.