When a number, or polynomial in algebra, can be expressed as the product of two numbers or polynomials, these are factors of the first. Expressing a number of polynomial as a product of its factors is known as factorising.
Example: 1, 2, 3, 4, 6 and 12 are all factors of 12.
Example: (x-1) and (x+4) are factors of (x2 + 3x - 4).