A trendline breakout happens when price moves decisively above a downward trendline or below an upward trendline, showing that the existing trend may be weakening and a new move could begin. It reflects a shift in market sentiment where buyers gain control after breaking resistance or sellers take control after breaking support. A valid breakout is stronger when supported by higher volume, strong candle closing beyond the trendline, and follow-through in the same direction. Traders often use trendline breakouts to identify fresh entry opportunities, while the broken trendline may later act as a new support or resistance level.