octagon An eight-sided polygon.

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

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

opposite  The numbers, which when added to a given number, yields a sum of 0. The opposite of a number n is denoted -n. Also called additive inverse.

opposite rays  Rays that have the same endpoint and together form a line.

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

order of operations  Rules for evaluating an expression: work first within parentheses; then calculate all powers, from left to right; then do multiplications or division, from left to right; then do additions or subtractions, from left to right.

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.

parallelogram A quadrilateral with two pairs of parallel sides.

parentheses ( )  Grouping symbols which indicate the order of operations that should be followed in evaluating an expression; the work inside them should be done first.

pattern  A general form for which there are many examples.

pentagon A five-sided polygon.

percent  % , times 1/100 or .01, one one-hundredth.

perimeter  The sum of the lengths of the sides of a polygon.

perimeter of a rectangle  2l + 2w where l is the length and w is the width of a rectangle.

perpendicular The name given to rays, segments, or lines that form right angles.

pi The ratio of the circumference of a circle to its diameter: approximately 3.1415926535 ... .

pie graph A graph in which information is represented using a circle that has been cut into sectors to show values of a particular category. Also called circle graph.

pint A unit of  capacity in the U.S system of measurements equal to one half quart.

place value  The numbers that each digit stands for in a decimal.

polygon  A union of segments connected end to end, such that each segment intersects exactly two others at its endpoint.

polynomial A monomial or a sum of any number of monomials.

positive integer  Any one of the numbers 1, 2, 3, ... . Also called natural numbers.

pound ( lb. )  A unit of weight in the U.S. system of measurement equal to 16 ounces.

power  The answer to a problem a^{^b}.

ppm Parts per million. This unit is frequently used in reporting the concentration of pollutants in water and air. It means that in a mixture, there is 1 part by mass of the subject material in 1 million parts of the mixture. Thus in water, 1 ppm corresponds to 1 g of pollutant per million g of water ( or, more accurately, of solution). By shifting the decimal point, you can see that this is equal to 0.001 g per 1000 g, or 1 mg/kg. Because 1 L of water weighs 1 kg, 1 ppm also corresponds to 1 mg/L.

preimage point  A point to which a transformation has been applied.

prime number  A positive integer whose only positive integer divisors are itself and 1.

probability A number from 0 to 1 which indicates how likely something is to happen.

problem  A question which you do not know how to answer.

product The resulting of doing a multiplication.

Product of Reciprocals Property If a and b are not zero, then 1/a times 1/b = 1/ab.

Property of Opposites  For any number n: n + -n = 0.

Property of Reciprocals For any nonzero number n: n times 1/n = 1.

proportion  A statement that two fractions are equal.

proportional thinking  The ability to get or estimate an answer to a proportion without going through the equation-solving process.

protractor  A instrument used for measuring angles.

Putting-together Model for Addition If a count or measure x is put together with another count or measure y in the same units, and there is no over-lap, then the result has count or measure x + y.

Pythagorean theorem  In a right triangle with legs a and b, and hypotenuse c, a² + b² = c².