O - Math terms glossary

oblique cone A cone whose axis is not perpendicular to its base.

oblique figure A 3-dimensional figure in which the plane of the base(s) is not perpendicular to its axis or to the planes of its lateral surfaces.

oblique line A line that is neither horizontal nor vertical.

obtuse angle   An angle whose measure is between 90° and 180°. Remember a 90° angle is called a right angle not an obtuse angle, and a 180° angle is called a straight angle not an obtuse angle.

obtuse triangle A triangle with an obtuse angle.

octagon An eight-sided polygon.

octahedron A polyhedron with eight faces.

odd node A node which is the endpoint of an odd number of arcs in a network.

one-dimensional A space in which all points are collinear. Basically a single line.

one-step proof A justified conclusion of a conditional requiring a single definition, theorem, or postulate.

open sentence  A sentence with variables that can be true or false, depending on what is substituted for the variables.

EX: x + 4 =16 is only true for x=12.

Op-op Property  For all real numbers n: -(-n) = n.

opposite  When a given number is added to its opposite you always get ZERO. The opposite of a number n is denoted -n. Also called additive inverse.

EX: 6 is the opposite of -6 because when you add them 6+-6=0.

opposite faces A pair of faces of a polyhedron whose planes are parallel to each other.

oppositely congruent Figures which are congruent and have opposite orientation.

opposite rays  Ray AB and Ray AC are opposite rays if and only if A is between B and C.

ordered pair  A pair of numbers ( x, y ) in which x is the first coordinate and y is the second coordinate.

ordered pair description of a vector The description of a vector as the ordered pair (a,b) where a is the horizontal component and b is the vertical component.

order of operations  Rules for evaluating an expression: work first within parentheses, then calculate all exponents, then do multiplication and division together as they appear in the problem from left to right; then do addition and subtraction as they appear from left to right.

NOTE: Many math teachers teach the saying,
"Please Excuse My Dear Aunt Sally" or PEMDAS. Each capital letter stands for an operation.
P~parentheses or all grouping symbols
E~exponents
M&D~ multiplication and division from left to right
A&S~ addition and subtraction from left to right

orientation The order of the designation of the vertices of a polygon, either clockwise or counterclockwise.

EX: figure ABCDEFG above has a clockwise orientation.

origin The point where the x and y axes intersect denoted by ( 0, 0 ).

ounce (oz) A unit of weight in the U.S. system of measurement equal to 1/16 of a pound. 1 ounce 29.574 milliliters.

overlapping figures Figures which have some interior points in common.

overlapping triangles Triangles that have interior points in common.