NBP stands for Negative Balance Protection.

These years, the phrase has been used more by many online FX brokers.

NBP literally protects traders from negative balance.

If an account balance is negative(minus) in a live trading account, the balance will be fixed to 0 automatically, thus traders do not need to cover the exceeded losses.

With NBP, traders will never lose more than they deposit, as in the loss is limited to the total deposit amount.

Recently(in 2017), this NBP is very common among online FX brokers though, there are still several brokers which do not support this condition.