The Relative Strength Index (RSI) is a momentum oscillator used in technical analysis to measure the speed and change of price movements. It helps identify overbought or oversold conditions in a security. Works best in ranging markets; can stay overbought/oversold in strong trends. Combine with other indicators (e.g., MACD, support/resistance). Adjustable periods: Shorter (e.g., 9) = more sensitive; longer (e.g., 21) = smoother. Use RSI to confirm trends, spot divergences, or signal potential reversals—never in isolation.