Risk per trade
The share of the account you are willing to lose on a single trade; the common recommendation is 0.5-1%, lower on prop firm accounts.
With 1% risk, 10 consecutive losses take about 10% of the account; with 5% risk, about 40%. Since losing streaks are unavoidable, risk per trade should be small enough to get through one.
A fixed percentage (1% of the current balance on every trade) shrinks risk as the account falls and grows it as the account rises; this gives natural protection.