What is considered "disposable income"?

"In chapter 13, 'disposable income' is income (other than child support payments received by the debtor) less amounts reasonably necessary for the maintenance or support of the debtor or dependents and less charitable contributions up to 15% of the debtor's gross income. If the debtor operates a business, the definition of disposable income excludes those amounts which are necessary for ordinary operating expenses." (11 U.S.C. § 1325(b)(2)(A) and (B))