The definition of "Velocity" in Nineyard is "a daily sales average".
The "General Velocity" column will show the effective daily average, which is used to calculate demand when Replenishing FBA and for Vendor Purchasing.
You can set specific time frames to be used for the "General Velocity" composition. Base it on the last 2, 7, 14, 30, 60, and 90 days. To get a daily average that reflects both, the recent history, as well as sudden trend changes, it's best that you build your velocity with a combination of multiple time frames.
In Settings>System Settings>Forecasting you can set different velocity settings for Purchasing and Replenishment.
In Replenish, the suggestion formula will consist of "General Velocity" multiplied by the "Min remaining days". The below example has a Velocity of 16.83, x 30 Min days = 505 suggested.
To ensure accuracy, velocities are being calculated using "In-Stock" days only. If your listing did not have stock for a day or more, Nineyard will look for more in-stock days prior to that.