Is return value of a Scheme "define" expression undefined? why?

[chris at seberino.org]

I remember reading somewhere but I can't find it now that
the return value of a Scheme define expression like (define pi 3.14)
is undefined.

mit-scheme seems to return the symbol which seems perfectly reasonable.

If it is undefined why is that so?  Wouldn't specifying it remove ambiguity and
avoid problems?

Chris