Some variety of structure, natural or artificial, that resists gravity and provides shelter from at least one of the elements.

Form

hey J James, you been around lately and I have just been missing you or?

as to the question, what do you think defines a building? structure, program or construction?

Isn't it as simple as just the result of the act of 'building', i.e. construction? I'd argue that program or formal consideration may not be necessary.

I agree with FRaC, I believe a building is defined through form. Function is always interchangeable which is proven by many a starchitect that you can insert program into the most outlandish of forms.

I think a better question might be:

How do you define Architecture? 

Architecture? I would say that when the intended purpose of a building becomes secondary to something more fundamental to the human or natural condition with regard to culture, art, or science, it is Architecture.  When "nothing can be either added or subtracted without comprimising its nature" it is great Architecture

the planning department.

I think a building is something that is constructed and provides useful space for its inhabitants.  A wheat silo is not a building and neither is a bridge, they are just structures

House designs are buildings and so are stadiums and libraries.