Proportionality

6 is proportional to 3 in that 6 is double of 3.

3 is proportional to 6 in that 3 is half of 6.

What value is proportional to another value if we can get from one Valley to the other using multiplication by a constant.

$y = 2x$

For the above equation, we may refer to the two as being a constant of proportionality.

We might even use a letter:

$y = mx$