Order block
The last opposite-direction candle(s) right before a strong move; the zone where institutional orders are assumed to have entered, used as an entry area on a return.
Bullish order block: the last down candle before a sharp rally. When price returns to this zone, buyers are expected to step back in. The zone's validity depends on the move out of it having created a break of structure (BOS).
The concept comes from the SMC/ICT literature; in classic terms it is close to a "supply/demand zone". Not every order block works; it is judged together with higher-timeframe direction and liquidity context.