The Relative Strength Index (RSI) is a momentum oscillator that measures the speed and change of price movements, typically used in technical analysis to identify overbought or oversold conditions in a market. The RSI ranges from 0 to 100 and is generally considered overbought when above 70 and oversold when below 30. Traders often use it to help make decisions on buying or selling securities, as it can signal potential reversals or continuation of trends.