Mike is right.
Unless a foundation (or room for that matter) is a perfect rectangle or a square (I know, same thing) many will default to 'irregular'. Some however, like myself, may state the foundation size in square feet (if it is available and documentable).
Either way, an 'irregular' foundation may be perfectly fine. On the other hand, it may "really" be irregular - In which case, I hope you have a good home inspector. ;)