Risk per trade
Risk per trade is the money you lose if the stop is hit, chosen before the entry, usually as a fixed share of the account such as 1%.
A fixed risk per trade keeps a losing streak survivable. Ten losses in a row at 1% of what is left leave 90.4% of the account; at 5% they leave 59.9%, which then needs a 67% gain to get back.
Learn more: the most common mistakes new traders make and the risk formula.