A leak is any pattern of deviation from optimal strategy that results in systematically repeated EV loss. Leaks can be technical (over-folding on certain board textures, incorrect sizing, bad calls in specific spots) or mental game related (tilt, hasty decisions when tired). Identifying leaks requires database analysis with poker tracking software or extensive solver study sessions. Fixing a single significant leak can dramatically improve winrate: a -2 BB/100 leak corrected for a player with +3 BB/100 winrate represents a 67% improvement in total performance.