The terms are used synonymous...though lateral float is less accurate as a general reference. If you draw a line perpendicular to your foot from the cleat outboard...lateral float is the tangent to the rotation of your foot about the cleat center. The front of your foot doesn't laterally displace much...but the rear does as your foot rotates about the cleat. Technically lateral float is not purely lateral but for small rotations aka all most pedals will permits, float can be construed as lateral and would be defined as a distance aka displacement and rotational float is described in degrees. Btw you simply correlate the two relationships with simple trig.