Max drawdown is the largest peak-to-trough decline a strategy or stock has experienced over a measurement window. A 30% max drawdown means at the worst point the position was down 30% from its prior high. The metric matters because most retail investors abandon strategies during drawdowns, locking in losses.
Computed as max over all (t1, t2) of (peak[t1] − trough[t2]) / peak[t1] for trough[t2] occurring after peak[t1]. Strategies with high Sharpe but very deep drawdowns are psychologically hard to hold — Long-Term Capital Management had Sharpe near 4 right before its 2-week 90% drawdown. APEX position-sizing via Kelly and conformal intervals is designed to keep individual-position drawdowns inside tolerable ranges.