Imagine it being a slippery word that somewhat anticipates the boat's planned (expected?) use, throws a dart at a speed that matches that expected use, and then matches things like fuel usage, comfort, noise levels, whatever... to that "darted" speed.
IOW, it depends to an extent on hull form, and expectation of how the user might use that hull form.
A planing hull might be expected to plane, so "cruising speed" for that hull is likely to be "on plane" at a reasonable mix of noise, comfort, fuel consumption, etc. (For us, that's about 19-21 kts.)
OTOH, a full displacement hull might be expected to putter along nicely, so cruise would well be an expected "hull speed." There's a formula for that, relative to a boats overall length at the waterline, hull speed is actually more like "the fastest you can go without trying to climb your own bow wave and quadrupling your fuel use" or something like that.
In both cases, "efficiency" is maybe in the eye of the beholder. For us, puttering along at displacement speeds is cheap (in the grand scheme of things) even though that's not the way the boat use was envisioned and even though others with the same style of boat might blast around at 20-25 kts. For the full displacement guys, usually fuel economy is best at speeds lower than that maximum theoretical hull speed (a slight modification to the same formula)... but then the FD boater may value arrival time (speed related) more than fuel savings.
IOW...
It depends.
-Chris