Expectancy
The average result per trade, after costs, usually expressed in R (multiples of the amount risked).
Expectancy = win rate × average win − loss rate × average loss − costs. A 45% win rate with 2R winners, 1R losers and 0.05R of costs gives 0.45 × 2 − 0.55 − 0.05 = +0.30R per trade.
If expectancy is zero or negative, no position size or exit rule turns it positive. Sizing then only changes how quickly you reach either the target or the drawdown. Measure it from a real sample of trades, and assume backtested figures are optimistic.
Related
General definition. Rules differ between firms and change over time; your firm’s current terms always take precedence.