When a strategy is tuned so tightly to the past that it only works on the past.
Overfitting is when you tweak a strategy until it fits your history almost perfectly, and along the way you fit the noise. The backtest looks incredible. Then it fails live, because the awesome hot streaks never repeat.
Every time you add a rule, a filter, or tweak a parameter to make the equity curve prettier, you risk fitting the past instead of the market. Test 200 parameter combos and keep the best one, and you've hand picked the luckiest version, not the truest. A backtest with 15 conditions on 300 trades isn't a strategy. This is simple: it wins beautifully in the sample and fails the moment it meets data it never saw.
Hold your data. Build on one slice, then test on a slice the strategy never touched, your out of sample data, and see if the edge survives. If the in sample makes +0.4R a trade and the out of sample makes +0.05R, that gap was fitting, not skill. Walk forward testing repeats the trick across the whole record, which is much harder to cheat on than a single split. More trades help too, an edge measured over 760 trades is far tougher to fake than one from 40.
Yes. Strip a system down to two rules and it can be too blunt to catch anything, robust but edgeless. The goal was never the simplest possible strategy. It's the simplest one that still holds up out of sample. And a little curve fitting is unavoidable, honestly, since you tuned something to end up here. The sin isn't fitting, it's fitting so hard you fool yourself into trading a backtest.