Quote from JoshDance:
and:
http://www.elitetrader.com/vb/showthread.php?threadid=31823
both agree with my definition. I don't think it has anything to do with upticks or downticks. A stock's volume falls into "UVOL" if that stock's current price is higher than yesterday's close. If it goes below by a penny, then that volume is subtracted from UVOL and added to DVOL. And vice versa.
Again, not an authority, but the best I can find in a quick search.
Quote from gmst:
If that is the case, then to make dvoluvold more useful, needs to be looked in the context of ES current vs ES close yday.
Quote from gmst:
yday 4 pm close was 1290.75, 415 close was 1292
Depending on which one you take, half gap fill comes out to be 1286.75 and 1286.25. So a safer target for a long here would be 1286 or maybe even 1285.75

at least for today.