Merriam-Webster is having a hard time with it. None of their definitions of rocker are relevant. However, sill is defined as “a strong horizontal member at the base of any structure, e.g., in the frame of a motor or rail vehicle”. But a rocker panel is defined as, “the portion of the body paneling of a vehicle that is situated *below *the doorsills of the passenger compartment”.
So they are trying to divide up the sill and rocker panel when in fact it is the combination of the two that gives structure support to the body.
And to further confuse things, in all the cars I have done such work on, there is an inner sill panel that carries most of the load and an outer panel that is both load bearing and forms the outer skin that you see.
Here is a TR3 inner sill, the structural support that the A and B pillars, the inner fenders and the floor are welded to and bolts to the frame. : http://the-roadster-factory.com/TRFpartsProjects.php
Here is an outer sill that spot welds onto the inner sill along the top and bottom flange: http://www.kas-kilmartin.com.au/TR009L%20TR3A%20-%2060000%20on%20Outer%20sill%20-%20rocker.JPG
I note that over the course of time the term “rocker” is now the opposite of the carriage’s use of the term. Where once the rocker is purposefully hidden and was not really structural today it is in view but does carry bending load.