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Our printable Math Dictionary words that start with O has the all the Math terms that begin with the letter O that I have found in the many years I have been using kids Math glossaries.

If you teach the vocabulary meaning of the prefixes ob- and oct-, the children will learn this list quickly.

**Oblate Spheroid** - an ellipsoid produced by rotating an ellipse through 360º about its minor axis.

**Oblique** - slanting or sloping not horizontal or vertical.

**Oblique Angle** - an angle whose measure is greater than 90 degrees.

**Oblique Coordinates** - a coordinate system in which the axes are not perpendicular.

**Oblique Lines** - those which approach each other, and will meet if sufficiently produced.

**Oblique Prism** - one whose sides do not form 90 degree angles with the bases.

**Oblique Triangles** - a triangle whose measure is greater than 90 degrees.

**Obtuse Angle** - an angle larger than 90º but smaller than 180º.

**Obtuse Triangle** - a triangle that has one angle that is greater than 90 degrees.

**Octagon** - an eight sided polygon.

**Octahedron** - a 3 dimensional geometric figure (a polyhedron) that is made up of 8 equilateral triangles.

**Octant** - any one of the 8 portions of space determined by the 3 coordinate planes.

**Odd Function** - a function f(x) is called an odd function if f(x)=-f(-x) for all x.

**Odd Number** - an integer that is not evenly divisible by 2.

**One to One** - a function f is said to be one to one if f(x)=f(y) implies that x=y.

**Onto** - a function f is said to map A onto B if for every b in B, there is some a in A such f(a)=b.

**Open Interval** - an interval that does not include its two endpoints.

**Optical Illusion** - a drawing or object that appears to have an effect that it does not really have.

**Order of Operations** - tells which order of operations used to solve math problems: (1) Parenthesis (2) Exponents (3) Multiplication & Division (4) Addition (5) Subtraction. (Please Excuse My Dear Aunt Sally.)

**Ordered Pair** - a set of two numbers in the form: (x, y).

**Ordinal Number** - a number indicating the order of a thing in a series.

**Ordinate** - the y-coordinate of a point in the plane.

**Origin** - the point at which the horizontal and vertical axes intersect, at zero (0,0).

**Orthic Triangle** - the triangle whose vertices are the feet of the altitudes of a given triangle.

**Orthocenter** - the point of intersection of the altitudes of a triangle.

**Outcome** - any one of the possible results of an experiment.

**Outcome Space** - the set of all possible outcomes of a given experiment.

**Outlier** - a data point (or points) that lie far outside most of the rest of the points in the data set.

**Outlier** - a number in a list of data that is distant from the other numbers in the list.

**Output** - the number or value that comes out from a process. P0=0, P1=1, and Pn=2Pn-1+Pn-2.

N * O * P * Q * R * S * T * U * V * W * X * Y * Z

I keep these definitions as a glossary in my binder.

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