Wikipedia currently says about duck-typing:
In computer programming with object-oriented programming languages, duck typing is a style of dynamic typing in which an object's current set of methods and properties determines the valid semantics, rather than its inheritance from a particular class or implementation of a specific interface.
It says about structural typing:
A structural type system (or property-based type system) is a major class of type system, in which type compatibility and equivalence are determined by the type's structure, and not through explicit declarations.
It contrasts structural subtyping with duck-typing as so:
[Structural systems] contrasts with ... duck typing, in which only the part of the structure accessed at runtime is checked for compatibility.
However, the term duck-typing seems to me at least to intuitively subsume structural sub-typing systems. In fact Wikipedia says:
The name of the concept [duck-typing] refers to the duck test, attributed to James Whitcomb Riley which may be phrased as follows: "when I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck."
So my question is: why can't I call structural subtyping duck-typing? Do there even exist dynamically typed languages which can't also be classified as being duck-typed?
Postscript:
As someone named daydreamdrunk on reddit.com so eloquently put-it "If it compiles like a duck and links like a duck ..."
Post-postscript
Many answers seem to be basically just rehashing what I already quoted here, without addressing the deeper question, which is why not use the term duck-typing to cover both dynamic typing and structural sub-typing? If you only want to talk about duck-typing and not structural sub-typing, then just call it what it is: dynamic member lookup. My problem is that nothing about the term duck-typing says to me, this only applies to dynamic languages.