Oblique line and a plane: A line and a plane that are neither parallel nor perpendicular.
Obtuse angle An angle whose measure is greater than 90 degrees and less than 180 degrees.
Obtuse triangle: A triangle having one obtuse angle.
Octagon: A polygon with 8 sides.
Octahedron: A polyhedron having eight faces. A regular octahedron is one of the five Platonic solids and has eight equilateral triangles as faces.
Odd number : Any integer that CANNOT be divided exactly by 2.
Odometer : An instrument (usually on the instrument panel of a car) that records distance traveled.
(A speedometer measures the current speed)
Ogive : It is also known as Frequency curve. It is used to find median of the data.
One-to-one function: A function where the inverse is also a function.
Operator : An Operator is a symbol (such as +, ×, etc) that represents an operation (i.e. you want to do something with the values).
Opposite rays: Two collinear rays whose intersection is exactly one point.
Opposite side in a right triangle: The side across from an angle. In a right triangle the hypotenuse is opposite the right angle and each leg is opposite one of the acute angles
Opposite transformation: A transformation of the plane that changes the orientation of a figure.
Order of Operations : The rules of which calculation comes first in an expression(PEDMAS).
Ordered pair: Two numbers that are used to identify the position of a point in a plane. The two numbers are called coordinates and are represented by (x, y).
Ordered triple: Three numbers that are used to identify the position of a point in space. The three numbers are called coordinates and are represented by (x, y, z).
Ordinal Number : A number that tells the position of something in a list.
Ordinate: The vertical coordinate of a two-dimensional rectangular coordinate system; usually denoted by y.
Orientation: The arrangement of the points, relative to one another, after a transformation; the reference made to the direction traversed (clockwise or counterclockwise) when traveling around a geometric figure.
Origin : The point in the Cartesian coordinate plane at which the horizontal and vertical axes intersect, designated by the ordered pair (0,0).
Orthocenter :The point of concurrence of the three altitudes of a triangle. Math dictionary o