Relative Strength Index. A technical analysis indicator which measures the magnitude of gains over a given time period against the magnitude of losses over that period. The equation is RSI = 100 - 100 / (1 + RS) where RS = (total gains / n) / (total losses / n) and n = number of RSI periods. The value can range from 1 to 100. Some technical analysts believe that a value of 30 or below indicates an oversold condition and that a value of 70 or above indicates an overbought condition.