A trendline breakout occurs when price decisively moves beyond a well-respected trendline that was previously acting as support or resistance, signaling a potential change in market direction or acceleration of the existing move. In a downtrend, a breakout above the descending trendline suggests selling pressure is weakening and buyers may be taking control; in an uptrend, a break below the ascending trendline indicates rising supply and possible trend reversal. A valid breakout is usually supported by strong candle closes beyond the trendline, increased volume, and follow-through price action, while false breakouts often lack these confirmations. Traders commonly wait for a break and retest of the trendline to improve reliability and manage risk effectively.