composite number A natural number that has at least one factor other than 1 and itself. denominator The bottom number of a fraction that tells how many equal parts are in the whole. divisibility test A rule that tells quickly whether dividing a number by another number can be done without leaving a remainder. divisible Can be divided by a number without leaving a remainder. For example, 20 is divisible by 4 because 20 ÷ 4 = 5 (no remainder). divisor The number that is being divided into the dividend in a division problem. In the problem 8 ÷ 2 = 4, 2 is the divisor. even number A whole number that is divisible by 2. exponential notation A notation that represents repeated multiplication using a base and an exponent. For example, 24 is notation that means 2 • 2 • 2 • 2. This notation tells you that 2 is used as a factor 4 times. 24 = 16. (Also called exponential form.) factor A number that is multiplied by another number or numbers to get a product. For example, in the equation 4 • 5 = 20, 4 and 5 are factors. factor pair A pair of numbers whose product is a given number. For example, 2 and 15 are a factor pair of 30 because 2 •15 = 30. Both 2 and 15 are factors of 30. factor tree A diagram showing how a number can be written as factors, and those factors written as a product of factors, and so on until only prime numbers are used. multiple Any number that has a given number as a factor. For example, 4, 8, 16, and 200 are multiples of 4, because 4 is a factor of each of these numbers. natural number The numbers 1, 2, 3, 4 and so on. Also called counting numbers. numerator The top number of a fraction that tells how many parts of a whole are being represented. prime factorization A number written as the product of its prime factors. prime number A natural number with exactly two factors: 1 and the number itself.