Bear in mind that generally a bug is something software developers are supposed to fix.  So the definition of a bug is based on what they want to fix.  For example, "working correctly more than 50% of the time is a feature we plan to release in future versions".  *Anything* can be defined as not being a bug by pretending the software was never intended to address that particular problem.  So, in practice, what constitutes a bug is a purely political consideration.

(As an aside, this cuts both ways.  To a client that doesn't have to pay for bug fixes but does have to pay for new development "it doesn't do some feature that I just thought of but which I've now decided is 100% implied by the things I did mention" is clearly a bug.)