The same definition as it is everywhere - a court rules that the parent is not fit. Its a subjective term with no clear definition. Its basically at the discretion of the court. Give us some examples of why you think your ex is unfit and we'll give you our best *guess* how a court would rule. But be aware - unless there is abuse, criminal neglect or prolonged incarceration involved, 'unfit' is usually just a matter of opinion and not a court order.