The Relative Vigor Index (RVI) is a popular and free Forex indicator used in the MT4 platform. The RVI is based on the idea that when the closing price of a currency pair is higher than the opening price, the closing price is considered to be more vigorous than the opening price. The RVI indicator is designed to measure the relative vigor of the closing price compared to the opening price. The RVI indicator can be used to identify potential trading opportunities in the Forex market and can be a useful tool for traders to determine the strength of a trend.