RelativeStrengthIndex

Commonly abbreviated as RSI.
Is a momentum indicator.
Was developed by Welles Wilder.
Is computed using close prices.
Returns one time series, which is the ratio of 14-period exponential moving averages for gains and losses rescaled to be between 0 and 100.
FinancialIndicator["RelativeStrengthIndex",n] uses a period-n moving average.
The first value in the time series occurs after periods.