What is the definition of "best L/D"?
Best $L/D$ is short hand for the airspeed where you have the best ratio of Lift to Drag, and is the same as $\textrm{max}\ L/D$.  

If you graph Airspeed -vs- the $L/D$ ratio, it will look something like this:



The peak point of the graph would be the "best" $L/D$ and represents the airspeed where the airplane is the most aerodynamically efficient.  For unpowered flight, it is the airspeed which will result in the best glide ratio.