Common Multiple

A multiple that is common to two or more numbers.

Example:

The multiples of 3 are: 3, 6, 9, 12, 15, 18, ... etc
The multiples of 5 are: 5, 10, 15, 20, 25, ... etc

So, a common multiple of 3 and 5 is 15 (and 30, 45, etc ..., as well)