velocity is measured in pixels per frame. It is basically a coordinate difference of track coordinate at current frame and previous one (no future prediction happens). It's not really most intuitive place for such a things, but historically the node was called this way.. Track velocity could be used to face effects like motion blur bu piping it to the vector blur node. Reviewers: campbellbarton Reviewed By: campbellbarton Subscribers: hype, sebastian_k Differential Revision: https://developer.blender.org/D1591