Factor: A number is the product of several numbers, and those numbers are the factors of this number. Multiplication: A number is the product of several numbers, and this number is a multiple of those books. Odd number: among natural numbers, numbers that are not multiples of 2 are called odd numbers. Even number: In natural numbers, numbers that are multiples of 2 are called even numbers. Prime number: If a number has only two factors: 1 and itself, it is called a prime number (or prime number). Composite number: A number is called a composite number if it has other factors besides 1 and itself.