It stands for volume momentum; a volume indicator that is calculated by finding the change in volume over a given period of time. If t is today and n is the number of days back (the observation period), then volume momentum (VM) is the difference between volume today (the end of observation period) and volume then (the start of observation period), and is given by:

VM_t = V_t – V_t-n

It is typical that the variance in volume is high from day to day, volume momentum suffers increased errors, prompting technical analysts to smooth the resulting volume momentum in order to have a reliable indicator.