Relative volume (often RVOL) compares a stock's current trading volume to its own average volume over a recent period. An RVOL of 3 means the stock is trading three times its usual activity.
Why it is a powerful filter
Unusual volume means something is happening — news, a catalyst, institutional interest. Breakout and momentum strategies rely on relative volume to separate stocks that are genuinely "in play" from the rest of the market that is just drifting. Research on the opening range breakout strategy finds that most of its edge comes precisely from this selectivity: trade only the few names that are unusually active.