CRITICAL WING
Guys, I've also come across this term with a few aircraft types, the one that comes to mind is the Dornier 228 turboprop..nothing to do with mach numbers .
It's described as having a "critical wing" because of it's streamlined shape making it more efficient / economic though nowhere near any critical mach number.
An aerodynamicist could likely tell you a lot more than me, but as I understand it, it's referred to a "critical" because it absolutely hates ice build up.
A big fat chunky wing of ..let's say an HS 748, has a well pronounced camber on its top surface, and so can carry quite a lot of ice build up before that camber shape changes significantly enough to seriously reduce th eamount of lift it produces.
A smaller sleeker wing with less camber, like that of the Do228, can only cope with a small amount of ice build up before its shape and therefore lift-producing capability starts to diminish.
But I have no idea how the designers calculate at which point such a wing can be described as "critical"