I define bat speed as how quick, in terms of split seconds the bat gets to the hitting zone. This is pretty much accomplished through the straight line principle. "The fastest way from one point to another is in a straight line".
We can break down the whole swing into the following components:
Preliminary Motion (Load, Stride, Etc)
Although the preliminary motion can greatly hinder the hitters ability to get the bat head to the contact point, the actual bat speed is from the end of the preliminary motion to the contact point. The approach.
Bat speed is measured in terms of fractions of seconds. The faster the bat speed, the longer the hitter can wait until she makes the decision of which to swing.
Velocity does play a role in bat speed too.