In general, a number or algebraic expression is a factor (or divisor) of another if there exists a number or algebraic expression of a specified type for which

A natural number is a factor of a natural number if there is a natural number such that

For example, is a factor of 12 because

A polynomial is divisible by a polynomial if there is a polynomial for which

For example, is a factor because .

A prime factor of a natural number is a factor of that is a prime number.

For example, the prime factors of are