Core terms
Relative volume (RVOL)
Volume for a period divided by the average volume for a comparable period in the past. An RVOL of 5 means five times the usual volume; tools differ on the period and the average they use.
RVOL answers "is this stock busier than usual?" The answer depends on the definition. Some tools compare today's volume so far with the average full-day volume, which understates RVOL early in the day. Others compare with the average volume up to the same time of day, which is fairer for intraday use. The averaging window, often 10 to 30 sessions, also changes the number.
RVOL measures attention, not direction. Heavy volume on a falling stock produces the same high RVOL as heavy volume on a rising one.
What traders should verify
Write down which RVOL your scanner uses (time-of-day or full-day, and how many days) before setting a threshold.
Do not conflate
Trader usage; no standard formula. A stock that rarely trades can show a huge RVOL on modest volume. Compare it with dollar volume.
Primary source
Trader usage: there is no official definition, and platforms and tools may calculate it differently.