A **trendline** is a straight line used in charts to represent the general direction or pattern of data over a specific time period. It helps to visually indicate whether the data is trending upwards, downwards, or staying relatively constant. Trendlines are widely used in technical analysis, particularly in financial markets, to identify trends in stock prices, currencies, or commodities. They can be drawn on various types of charts like line charts, bar charts, or candlestick charts to predict future movements based on past performance.