The speed of change of the input value. The output is only updated when the input is changed and will be constant if the input is constant. The velocity can be negative if the input value is decreasing.
Velocity
The difference between the previous two changes in the input value, divided by the time elapsed between the changes.
Example
If the input starts at 10 and is changed to 20 two seconds later then the velocity will be (20-10)/2 which is equal to 5.