A number $a$ is a factor of another number $b$ if $a$ divides $b$ evenly and there is no remainder. This also means that the resulting number $c$ is such that the product of $a$ and $c$ equals $b.$

The numbers $a$ and $c$ are factors of $b$ because of the Commutative Property of Multiplication. For this reason, each multiplicand in a multiplication expression is called a factor. A number can have many factors.

Number	Factors
$48$	$1,$ $2,$ $3,$ $4,$ $6,$ $8,$ $12,$ $16,$ $24,$ $48$
$21$	$1,$ $3,$ $7,$ $21$
$9$	$1,$ $3,$ $9$

Factors can also be extended to negative numbers because, for example, $\text{-}2\times \text{-}24=48.$

Number	Negative Factors
$48$	$\text{-}1,$ $\text{-}2,$ $\text{-}3,$ $\text{-}4,$ $\text{-}6,$ $\text{-}8,$ $\text{-}12,$ $\text{-}16,$ $\text{-}24,$ $\text{-}48$
$21$	$\text{-}1,$ $\text{-}3,$ $\text{-}7,$ $\text{-}21$
$9$	$\text{-}1,$ $\text{-}3,$ $\text{-}9$

Factors are not limited to numerical expressions. When multiplying algebraic expressions, each expression is also a factor.