As a lay person I apologise if this is the wrong forum please let me know if it isn't.
In flat taxation regime you pay x% of some value (income for arguments sake). This remains at x% as value decreases or increases.
In a progressive taxation regime you pay a varying % dependant upon some variables bases on a function of the 'some value'.
So for example the rate may be y% (where y < x) for where some value is less than the above or possibly be z% (where z > x) for some value is greater than in the able example.
What I'm trying to find is if there is name for a function where a proportion varies according to the size of the whole. If you see what I mean.