The nominal interest rate (TIN in Spain) is the simple interest rate applied to a loan, account or deposit, without considering fees or compounding frequency. It is the number most loudly displayed in adverts.
That is why regulators force it to be shown alongside the APR: APR is the number actually useful for comparison. If you only see the nominal rate, you do not know the real cost or yield.
Example: a 4% nominal deposit with quarterly compounding and a maintenance fee can show a very different APR. Always look at APR first.