A plane hull is considered a monocoque structure as well. That's not one piece for sure. It's many sheets riveted together to form a single shell.
Depends on how it's done and how the hull relates to the rest of the aircraft.
Same goes for the hull of a ship, boat, yacht, etc.
However, turning the problem upside down:
Would you consider a bicycle made of an aluminium main triangle and a carbon chainstay a monocoque frame?
Why do you think the people at pedalforce stress the fact that the frame they offer is made as they describe and as such qualifies as a true monocoque frame as opposed to the cobbled up constructions using alu or whatever non-carbon materials as structure carrying lugs?
A frame made of several boxes of carbon could be said to have a monocoque main triangle, a mono rear stay etc. But that would not per definition make it a monocoque frame.
From a dictionary:
a.,n. (denoting) aircraft structure in which the outer skin carries all or most of the torsional and bending stresses; (denoting) car structure in which the body is integral with and shares the stresses with the chassis.
© From the Hutchinson Encyclopaedia.
Helicon Publishing LTD 2000.
All rights reserved
And another one from Wikipedia:
Monocoque (French for "single shell") or unibody is a construction technique that uses the external skin of an object to support some or most of the load on the structure. This is as opposed to using an internal framework that is then covered with a non-load-bearing skin. Monocoque construction was first widely used in aircraft, starting in the 1930s, and is the predominant automobile construction technology today.
So when alu lugs are used to hold the carbon "hull/frame" together it would still qualify as a "monocoque" structure?
I suppose it all depends where you want to draw the line and how flexible
you want to be but in the bicycle trade the carbon frame using the lugged construction technique won't be sold from one professional to another as a monocoque frame. That much I can tell you with absolute certainty.
However, you may well see that same lugged frame being advertised as a monocoque offering.
Now that's one I most certainly won't buy because I'll damn well be doubledutched. See the difference?