A number that divides an exact number of times into another number is said to be a **factor**, **sub-multiple** or
**measure** of the latter number. For example, 15 = 3 × 5, so both 3 and 5 are factors of 15.

A number that, when divided by another number, leaves no remainder, is said to be a **multiple** of the latter number. For example, 15 ÷ 3 = 5, so 15 is a multiple of 3 (and 5).

By the Fundamental Theorem of Arithmetic, every natural number greater than 1 can be factored into prime numbers in one way and one way only.

The **Least Common Multiple** of two or more numbers is the smallest number that is a multiple of each number.

The **Greatest Common Divisor** (also known as **Greatest Common Factor** or **Greatest Common Measure**) of two or more numbers is the greatest number contained an exact number of times in each number.