Greatest common divisor (GCD)

The "greatest common divisor" of two integers $x$ and $y$, denoted $\text{gdc}(x,y)$ is the largest natural number that divides both of the integers.

For example, $\text{gdc}(8,12)$ is 4, because:

Two numbers such that the greatest common divisor is 1.

TODO wtf is a "totient"? Where else is that word used besides in this concept?