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.
For the above equation, we may refer to the two as being a constant of proportionality.
We might even use a letter:
Vectoreverything 