The Relative Strength Index (RSI) is a technical analysis indicator developed by J. Welles Wilder Jr. in 1978. It measures the magnitude of recent price changes to evaluate overbought or oversold conditions in a market, typically on a scale from 0 to 100. Readings above 70 often indicate overbought conditions, while readings below 30 suggest oversold levels. RSI is widely used by traders to assess momentum and potential trend reversals.