What does protein "purification factor" mean?

The purification factor is a number that tells you and others basically how pure your protein is. I know that sounds simple, but that's what it is. How much "enrichment" you have. How well did you get rid of all other extraneous protein. That's why for the enzyme you use specific acitivity. If you did it correctly, the specific acitity has units of protein in the denominator. So as you get rid of extraneous protein, but don't lose enzyme activity, the specific activity goes up. So, if the total amount of protein was cut in half, the specific activity would double, and the purification factor would be 2. Hope this helps.