A trendline breakdown occurs when price decisively falls below an upward-sloping trendline that was acting as support, indicating a possible shift from an uptrend to weakness or a downtrend. It shows that buyers are losing control and sellers are gaining strength, often accompanied by increased volume or strong bearish candles. This breakdown suggests that demand at higher levels is drying up, and the market may move toward lower support zones. Traders usually wait for confirmation—such as a close below the trendline or a retest failure—before expecting further downside.