We never define what a "virtual function" is.

Starting point for wording (as discussed on core reflector): prepend to [class.virtual]p1:

"A non-static member function is a virtual function if it is first declared with the keyword virtual or if it overrides a virtual member function declared in a base class (see below)."

Additional cleanups:

In [class.virtual]p2, strike the "is also virtual (whether or not it is so declared) and it", since that follows from the definition of "virtual function".

Move the second sentence of footnote 115 to a new footnote 115a attached to the newly added sentence in [class.virtual]p1, and replace the word "legal" with "well-formed" or "valid". Replace "has empty semantics" with "has no semantic effect".