[danaris]

I'm open to a different descriptor.

From where I sit, "income after tax" doesn't really seem to fit as the meaning of "disposable income", though I suppose I can see why it would look that way to an office like ONS.

Now, to be clear, "where I sit" is in the US, and I've never seen "disposable income" be given an official usage like that over here; its only meaning that I've known is, much more colloquially, just what I said: the amount of money you have left over each $timePeriod after paying for the basic necessities of life. (How one defines those, specifically, varies greatly from person to person.)