What is the meaning of the field for "Profitability decline per year" in bitcoinx' profitability calculator? In the Bitcoin mining profitability calculator at bitcoinx.com/profit, there is a field with the description "Profitability decline per year". What does this field mean in regard to the calculation?

At a guess, I'd say, if it's set to 0.01 the projected income for a year from now will be 1% of today's projected income.
I agree that it isn't worded very well. It seems to be a number r such that the revenue n years from now will be r^n times the revenue today, so basically what you said.

To empirically verify this, set electricity and hardware cost to 0 (so that profits equal revenue), time frame in months to 12 (for a full year), and profitability decline to 1. You should get $1,366 or so. If you change it to 24 months, you get $2,732 - exactly double the previous. On the other hand, with a profitability decline of 0.2, you get $679 in 12 months and $814 in 24 months, which is $135 (or 20%) more.