From what I've learned from some mechanics and my own experience, it's
the lowest speed at which the boat remains on plane AND the motor can
be trimmed so the angle of thrust is parallel, or very close to
parallel, to the surface, i.e. horizontal. If the motor's trimmed up
or down off-parallel, it's wasting energy pushing in the wrong
direction (any direction besides forward).

Sometimes surface conditions can interfere with this principle. For
example, you might have to trim the motor out (pushing the bow down)
slightly to prevent violent porpoising when the surface is rough. If
it's absolutely glass-smooth, then of course that is a different
situation.

Ron M.