Maximum Drawdown
Maximum Drawdown (Max DD) is the largest observed loss from a portfolio peak to a subsequent trough before a new peak is set.
The math
MDD = max over all t of [(V_peak(t) − V_trough(t)) / V_peak(t)]V_peak(t) = highest portfolio value up to time t, V_trough(t) = lowest value after that peak before recovery.
Why it matters
Max DD answers the question a live trader really cares about: "If I had run this strategy, what was the worst I could have felt?" A strategy with a 60% Max DD requires a 150% recovery just to break even. Magdon-Ismail & Atiya (2004) show the distribution of Max DD under a random walk, giving a baseline for how much drawdown is expected by chance.
Max DD is path-dependent and backward-looking — it tells you the worst case that already happened, not the worst case that could happen. A short backtest can underestimate true Max DD significantly. It also ignores how long the drawdown lasted (recovery time is a separate metric).
Published source
Magdon-Ismail, M. & Atiya, A. F. (2004). "Maximum Drawdown." Risk Magazine, 17(10), 99–102.